I have recorded and posted to http://www.unizor.com some material about infinity. More precisely, about different infinities. I've placed it into a new topic "Quantity" in the Math Concepts menu item. There are a few mini-theorems and exam problems as well.

Welcome to Unizor, the road to infinity!

## Sunday, November 27, 2011

## Wednesday, November 16, 2011

### Problems in Geometry Series 3

I have uploaded a new lecture to http://www.unizor.com/ with the following geometrical problems.

1. Prove that in an equilateral triangle sum of distances of any internal point to all three sides is constant and is equal to an altitude of a triangle.

2. Prove that a parallelogram with congruent diagonals is a rectangle.

3. Prove that a parallelogram with perpendicular diagonals is a rhombus.

4. Prove that a parallelogram with a diagonal being an angle bisector is a rhombus.

5. Given a rhombus. From a point of intersection of its diagonals we dropped perpendiculars to all four sides. Prove that points of intersection of these perpendiculars with sides form a rectangle.

6. Prove that angle bisectors of a rectangle form a square.

7. Given a square

8. Given a square

9. What condition should a quadrangle satisfy, if a new quadrangle with vertices at midpoints of each its side form

(a) parallelogram,

(b) rhombus,

(c) rectangle,

(d) square.

1. Prove that in an equilateral triangle sum of distances of any internal point to all three sides is constant and is equal to an altitude of a triangle.

2. Prove that a parallelogram with congruent diagonals is a rectangle.

3. Prove that a parallelogram with perpendicular diagonals is a rhombus.

4. Prove that a parallelogram with a diagonal being an angle bisector is a rhombus.

5. Given a rhombus. From a point of intersection of its diagonals we dropped perpendiculars to all four sides. Prove that points of intersection of these perpendiculars with sides form a rectangle.

6. Prove that angle bisectors of a rectangle form a square.

7. Given a square

**. Let points***ABCD***,***A'***,***B'***and***C'***be midpoints of sides***D'***,***CD***,***DA***and***AB***correspondingly. Prove that segments***BC***,***AA'***,***BB'***and***CC'***form a square with each side having a length of 2/5 of the length of each of these segments.***DD'*8. Given a square

**. Let points***ABCD***,***A'***,***B'***and***C'***be positioned on sides***D'***,***AB***,***BC***and***CD***correspondingly, such that segments***DA***,***AA'***,***BB'***and***CC'***are congruent. Prove that***DD'***is a square.***A'B'C'D'*9. What condition should a quadrangle satisfy, if a new quadrangle with vertices at midpoints of each its side form

(a) parallelogram,

(b) rhombus,

(c) rectangle,

(d) square.

### Problems in Geometry Series 2

I have uploaded a new lecture to http://www.unizor.com/ with the following problems.

1. Prove that a median to a hypotenuse in the right triangle is equal to half of that hypotenuse.

Hint: extend the median through hypotenuse by its length.

2. Prove a converse theorem, that if a median in a triangle is equal to half a side it falls on, then an angle, where it starts, is the right angle.

3. Prove that in a right triangle a median and an altitude to its hypotenuse form an angle, equal to difference between this triangle's acute angles.

Hint: use a previous theorem.

4. Given triangle Δ

5. Given an angle ∠

6. Prove that any segment, that connects two bases of a trapezoid, is divided by a median in two congruent parts.

7. Given a triangle Δ

8. Straight lines are drawn through all three vertices of a triangle, forming another (bigger) triangle. Prove that this bigger triangle is divided by sides of a smaller one into 4 triangles, each congruent to a small triangle and each side of a bigger triangle is twice as big as parallel to it side of a smaller triangle.

9. Prove that in an isosceles triangle sum of two distances from any point on a base to two legs is constant and is equal to an altitude from any of two end points of a base to a leg.

10. Change a condition of a previous theorem to use a point on continuation of a base (outside of a triangle). Formulate a theorem in this case and prove it.

1. Prove that a median to a hypotenuse in the right triangle is equal to half of that hypotenuse.

Hint: extend the median through hypotenuse by its length.

2. Prove a converse theorem, that if a median in a triangle is equal to half a side it falls on, then an angle, where it starts, is the right angle.

3. Prove that in a right triangle a median and an altitude to its hypotenuse form an angle, equal to difference between this triangle's acute angles.

Hint: use a previous theorem.

4. Given triangle Δ

**. Segment***ABC***bisects angle ∠***AD***(point***BAC***lies on side***D***). Straight line through point***BC***is parallel to side***D***, intersecting side***AC***at point***AB***. Straight line through point***E***is parallel to side***E***, intersecting side***BC***at point***AC***. Prove that segments***F***and***AE***are congruent.***CF*5. Given an angle ∠

**. Inside it an angle ∠***MXN***is positioned in such a way that***PYQ***is parallel to***MX***,***PY***is parallel to***NX***and distance between***QY***and***MX***is equal to distance between***PY***and***NX***. Prove that a bisector of angle ∠***QY***is a bisector of angle ∠***MXY***.***PYQ*6. Prove that any segment, that connects two bases of a trapezoid, is divided by a median in two congruent parts.

7. Given a triangle Δ

**. Let vertex***ABC***be at the top and side***B***be a base. Let point***AC***be an intersection of two bisectors of angles at the base. Straight line through point***X***parallel to a base intersects its left side***X***and right side***AB***at points***BC***and***M***correspondingly. Prove that segment***N***is equal to a sum of segments***MN***and***AM***.***CN*8. Straight lines are drawn through all three vertices of a triangle, forming another (bigger) triangle. Prove that this bigger triangle is divided by sides of a smaller one into 4 triangles, each congruent to a small triangle and each side of a bigger triangle is twice as big as parallel to it side of a smaller triangle.

9. Prove that in an isosceles triangle sum of two distances from any point on a base to two legs is constant and is equal to an altitude from any of two end points of a base to a leg.

10. Change a condition of a previous theorem to use a point on continuation of a base (outside of a triangle). Formulate a theorem in this case and prove it.

### Problems in Geometry Series 1

I have uploaded a new lecture to http://www.unizor.com/ with the following geometrical problems.

1. Construct a triangle by two sides and a median to the third side.

2. Construct a quadrangle by all four sides and a median segment connecting midpoints of opposite sides.

Hint: use parallel shift.

3. Construct a trapezoid by one of its interior angles, two diagonals and a median (segment between midpoints of two non-parallel sides).

Hint: use parallel shift.

4. Construct a quadrangle by three sides and two interior angles adjacent to one of known sides.

Hint: use parallel shift.

5. Construct a trapezoid by its two bases and two legs.

Hint: use parallel shift.

6. Construct a quadrangle by its four sides, provided one diagonal bisects one angle it is connected to.

Hint: use symmetry relative to an axis.

7. A billiard table has a rectangular shape. There are two balls on it in some position. Determine direction of the movement of one ball in such a way, that, reflecting from all four sides, it will hit the second ball.

Hint: use symmetry relative to table borders on each side.

1. Construct a triangle by two sides and a median to the third side.

2. Construct a quadrangle by all four sides and a median segment connecting midpoints of opposite sides.

Hint: use parallel shift.

3. Construct a trapezoid by one of its interior angles, two diagonals and a median (segment between midpoints of two non-parallel sides).

Hint: use parallel shift.

4. Construct a quadrangle by three sides and two interior angles adjacent to one of known sides.

Hint: use parallel shift.

5. Construct a trapezoid by its two bases and two legs.

Hint: use parallel shift.

6. Construct a quadrangle by its four sides, provided one diagonal bisects one angle it is connected to.

Hint: use symmetry relative to an axis.

7. A billiard table has a rectangular shape. There are two balls on it in some position. Determine direction of the movement of one ball in such a way, that, reflecting from all four sides, it will hit the second ball.

Hint: use symmetry relative to table borders on each side.

## Monday, October 10, 2011

### Construction Problems in Geometry

Construction problems in geometry are extremely important. They help to develop student's creativity, logical thinking and ability to combine analysis of a problem at hand with synthesis of existing methods into concrete solution. Solving problems is the most important purpose of UNIZOR.COM. Please examine these problems and try to solve them. The accompanying lecture contains all the solutions, but watch it only after you spent sufficient amount of time trying to solve these problems yourself.

1. Construct a sum of two angles.

In details, this construction problem has the following meaning, applicable to other problems as well (so, we will not repeat it).

Given two angles and a ray somewhere on a plane.

Using a given ray as one leg, construct another ray from the same starting point such that these two rays (one given and one constructed) form an angle, which has a measure equal to a sum of measures of two given angles.

2. Construct a difference between a bigger and a smaller angle.

3. Given a sum and a difference between two angles, construct these angles.

4. Divide an angle into 4, 8 and 16 congruent parts.

5. Construct a triangle by its two sides and an angle opposite to a bigger side.

Is it always a unique triangle? Analyse the number of solutions of this problem.

6. Construct a triangle by its two sides and an angle opposite to a smaller side.

Is it always a unique triangle? Analyse the number of solutions of this problem.

7. Construct an isosceles triangle by a base (a side, generally speaking, non-congruent to two others) and one of two other sides.

8. Construct an isosceles triangle by a base (a side, generally speaking, non-congruent to two others) and one of two congruent angles (usually called angles at the base or base angles).

9. Construct an isosceles triangle by one of two congruent sides and an angle between congruent sides (usually called an angle at the vertex).

10. Construct an isosceles triangle by one of two congruent sides and a base angle it forms with a base (a side, generally speaking, non-congruent to two others).

11. Construct a right triangle by its two legs (i.e. sides, perpendicular to each other, sometimes called also catheti).

12. Construct a right triangle by one of its legs (also known as cathetus) and a hypotenuse.

13. Construct a right triangle by a cathetus and an acute angle it forms with a hypotenuse.

14. Construct an isosceles triangle by an altitude to a base and one of two congruent sides.

15. Construct an isosceles triangle by an altitude to a base and an angle opposite to a base (i.e. vertex angle).

16. Construct an isosceles triangle by a base and an altitude towards one of two congruent sides.

17. Construct a right triangle by a hypotenuse and an acute angle it forms with one of catheti.

18. Given an angle and a point inside it. Construct a straight line crossing this point and cutting congruent segments from sides of a given angle.

19. Construct two segments by their given sum and difference.

20. Divide a given segment into 4, 8 and 16 congruent parts.

21. Given a straight line and two points anywhere on a plane. Construct a point on a given line equidistant to two given points.

22. Construct a point equidistant to three vertices of a given triangle.

23. Given an angle and a straight line that crosses both its sides. Construct a point on this line equidistant to two sides of a given angle.

24. Construct a point equidistant to three sides of a given triangle.

25. Given a straight line

26. Construct a right triangle by one of its catheti and a sum of hypotenuse and the other cathetus.

27. Construct a triangle by a side, an angle it makes with another side and a sum of two other sides.

28. Given an angle and two points, one on each side of an angle. Construct a point equidistant from both sides of an angle and, at the same time, equidistant from two given points on angle's sides.

Notice, that distances to angle's sides should be equal to each other, but not necessarily equal to distances to given points.

1. Construct a sum of two angles.

In details, this construction problem has the following meaning, applicable to other problems as well (so, we will not repeat it).

Given two angles and a ray somewhere on a plane.

Using a given ray as one leg, construct another ray from the same starting point such that these two rays (one given and one constructed) form an angle, which has a measure equal to a sum of measures of two given angles.

2. Construct a difference between a bigger and a smaller angle.

3. Given a sum and a difference between two angles, construct these angles.

4. Divide an angle into 4, 8 and 16 congruent parts.

5. Construct a triangle by its two sides and an angle opposite to a bigger side.

Is it always a unique triangle? Analyse the number of solutions of this problem.

6. Construct a triangle by its two sides and an angle opposite to a smaller side.

Is it always a unique triangle? Analyse the number of solutions of this problem.

7. Construct an isosceles triangle by a base (a side, generally speaking, non-congruent to two others) and one of two other sides.

8. Construct an isosceles triangle by a base (a side, generally speaking, non-congruent to two others) and one of two congruent angles (usually called angles at the base or base angles).

9. Construct an isosceles triangle by one of two congruent sides and an angle between congruent sides (usually called an angle at the vertex).

10. Construct an isosceles triangle by one of two congruent sides and a base angle it forms with a base (a side, generally speaking, non-congruent to two others).

11. Construct a right triangle by its two legs (i.e. sides, perpendicular to each other, sometimes called also catheti).

12. Construct a right triangle by one of its legs (also known as cathetus) and a hypotenuse.

13. Construct a right triangle by a cathetus and an acute angle it forms with a hypotenuse.

14. Construct an isosceles triangle by an altitude to a base and one of two congruent sides.

15. Construct an isosceles triangle by an altitude to a base and an angle opposite to a base (i.e. vertex angle).

16. Construct an isosceles triangle by a base and an altitude towards one of two congruent sides.

17. Construct a right triangle by a hypotenuse and an acute angle it forms with one of catheti.

18. Given an angle and a point inside it. Construct a straight line crossing this point and cutting congruent segments from sides of a given angle.

19. Construct two segments by their given sum and difference.

20. Divide a given segment into 4, 8 and 16 congruent parts.

21. Given a straight line and two points anywhere on a plane. Construct a point on a given line equidistant to two given points.

22. Construct a point equidistant to three vertices of a given triangle.

23. Given an angle and a straight line that crosses both its sides. Construct a point on this line equidistant to two sides of a given angle.

24. Construct a point equidistant to three sides of a given triangle.

25. Given a straight line

**and two points***PQ***and***M***outside it, but on the same side from it. Construct a point***N***on the line***X***such that angles ∠***PQ***and ∠***PXM***are congruent.***QXN*26. Construct a right triangle by one of its catheti and a sum of hypotenuse and the other cathetus.

27. Construct a triangle by a side, an angle it makes with another side and a sum of two other sides.

28. Given an angle and two points, one on each side of an angle. Construct a point equidistant from both sides of an angle and, at the same time, equidistant from two given points on angle's sides.

Notice, that distances to angle's sides should be equal to each other, but not necessarily equal to distances to given points.

### Geometrical Problems

Finally, I've reached a point when I can start introducing problems in Geometry. Problems below are, mostly, from more than 100 years old textbook on geometry by a famous Russian math teacher Kiselev.

Solving problems is the most important purpose of our site UNIZOR.COM. Please examine these problems and try to solve them. The accompanying lecture contains all the solutions, but watch it only after you spent sufficient amount of time trying to solve these problems yourself.

1. Prove that in an isosceles triangle two medians to congruent sides are congruent.

2. Prove that in an isosceles triangle two angle bisectors to congruent sides are congruent.

3. Prove that in an isosceles triangle two altitudes to congruent sides are congruent.

4. Prove that segments of two perpendicular bisectors to two congruent sides of an isosceles triangle between midpoint of one side and crossing of its bisector with another side are congruent.

More precisely, consider an isosceles triangle Δ

5. Prove that a line, perpendicular to an angle bisector, cuts from two rays forming this angle congruent segments, assuming that an angle is less that 180 degrees.

In details, let

6. Prove that median

Notice, that distance from a point to a line is measured as a length of a perpendicular dropped from this point to a line.

7. Prove that the length of median

Hint: extend the median beyond side

8. Prove that sum of lengths of all three medians of a triangle is less than its perimeter, but greater than its halp-perimeter.

Hint: use the triangle inequality and the previous problem.

9. Prove that sum of lengths of two diagonals of a quadrangle is less than its perimeter, but greater than its half-perimeter.

10. Given an angle ∠

Prove that crossing segments

Notice, that this theorem implies an easy way to construct an angle bisector.

11. Given a sraight line

Prove that there exists a point

12. Given a straight line

13. Given an accute angle ∠

Solving problems is the most important purpose of our site UNIZOR.COM. Please examine these problems and try to solve them. The accompanying lecture contains all the solutions, but watch it only after you spent sufficient amount of time trying to solve these problems yourself.

1. Prove that in an isosceles triangle two medians to congruent sides are congruent.

2. Prove that in an isosceles triangle two angle bisectors to congruent sides are congruent.

3. Prove that in an isosceles triangle two altitudes to congruent sides are congruent.

4. Prove that segments of two perpendicular bisectors to two congruent sides of an isosceles triangle between midpoint of one side and crossing of its bisector with another side are congruent.

More precisely, consider an isosceles triangle Δ

**with congruent sides**

*ABC***and**

*AB***. Draw perpendicular bisectors of its congruent sides through midpoint**

*BC***of side**

*M***and midpoint**

*AB***of side**

*N***. Point**

*BC***is a crossing point of the perpendicular bisector of side**

*P***with side**

*AB***. Point**

*BC***is a crossing point of the perpendicular bisector of side**

*Q***with side**

*BC***. Prove that segments**

*AB***and**

*MP***are congruent.**

*NQ*5. Prove that a line, perpendicular to an angle bisector, cuts from two rays forming this angle congruent segments, assuming that an angle is less that 180 degrees.

In details, let

**and**

*AB***be two rays forming angle ∠**

*AC***, let**

*BAC***be a bisector of this angle and**

*AM***- any point on this bisector. Draw a perpendicular to**

*P***that croses it at point**

*AM***. This same perpendicular crosses rays**

*P***at point**

*AB***and**

*X***at point**

*AC***. Prove that segments**

*Y***and**

*AX***are congruent.**

*AY*6. Prove that median

**of a triangle Δ**

*AM***from vertex**

*ABC***to an opposite side**

*A***is equidistant from vertices**

*BC***and**

*B***.**

*C*Notice, that distance from a point to a line is measured as a length of a perpendicular dropped from this point to a line.

7. Prove that the length of median

**of a triangle Δ**

*AM***from vertex**

*ABC***to an opposite side**

*A***is less than half sum of lengths of sides**

*BC***and**

*AB***in lies in between.**

*AC*Hint: extend the median beyond side

**by doubling its length to point**

*BC***and consider triangle Δ**

*D***.**

*ABD*8. Prove that sum of lengths of all three medians of a triangle is less than its perimeter, but greater than its halp-perimeter.

Hint: use the triangle inequality and the previous problem.

9. Prove that sum of lengths of two diagonals of a quadrangle is less than its perimeter, but greater than its half-perimeter.

10. Given an angle ∠

**and two points on each side:**

*PMQ***and**

*A***on side**

*B***and**

*MP***and**

*A'***on side**

*B'***such that corresponding pairs of segments are congruent:**

*MQ***is congruent to**

*MA***and**

*MA'***is congruent to**

*MB***.**

*MB'*Prove that crossing segments

**and**

*AB'***cross on a bisector of angle ∠**

*A'B***.**

*PMQ*Notice, that this theorem implies an easy way to construct an angle bisector.

11. Given a sraight line

**and two pairs of points symmetrical relative to this line: points**

*PQ***and**

*A***are symmetrical relative to**

*A'***, points**

*PQ***and**

*B***are symmetrics too relative to the same line**

*B'***.**

*PQ*Prove that there exists a point

**on line**

*M***equidistant from all four points**

*PQ***,**

*A***,**

*A'***and**

*B***.**

*B'*12. Given a straight line

**and two points**

*PQ***and**

*A***on the same side from it. Find a point**

*B***on line**

*M***such that the sum of lengths of segments**

*PQ***and**

*AM***is minimal.**

*MB*13. Given an accute angle ∠

**and point**

*PMQ***inside it. Find point**

*A***on side**

*X***of this angle and point**

*MP***on its other side**

*Y***such that a perimeter of triangle Δ**

*MQ***is minimal.**

*AXY*## Wednesday, August 10, 2011

UNIZOR provides the framework for that change; a place for the gathering of what education should be; a place for delivering greater value at a fraction of the cost. It sets the path for eventual replacement of obsolete educational methodologies.

UNIZOR allows parents to take control over the educational process by enrolling their children in particular programs, checking their progress and authorizing marking a particular lesson as "completed".

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