CBSE Class 10 Mathematics Triangles Notes Set A

Class-10

Chapter 6: Triangles

Chapter Notes

Top Definitions

1. Two geometrical figures are called congruent if they superpose exactly on each other that is they are of same shape and size.

2. Two figures are similar, if they are of the same shape but of different size.

3. Basic Proportionality Theorem (Thales Theorem): If a line is drawn parallel to one side of a triangle to intersect other two sides in distinct points, the other two sides are divided in the same ratio.

4. Converse of BPT: If a line divides any two sides of a triangle in the same ratio then the line is parallel to the third side.

5. A triangle in which two sides are equal is called an isosceles triangle.

6. AAA (Angle-Angle-Angle) similarity criterion: If in two triangles, corresponding angles are equal, then their corresponding sides are in the same ratio (or proportion) and hence the two triangles are similar.

7. Converse of AAA similarity criterion: If two triangles are similar, then their corresponding angles are equal.

8. SSS (Side- Side- Side) similarity criterion: If in two triangles, sides of one triangle are proportional to (i.e., in the same ratio of) the sides of the other triangle, then their corresponding angles are equal and hence the two triangles are similar.

9. Converse of SSS similarity criterion: If two triangles are similar, then their corresponding sides are in constant proportion.

10. SAS (Side-Angle-Side) similarity criterion: If one angle of a triangle is equal to one angle of the other triangle and the sides including these angles are proportional, then the two triangles are similar.

11. Converse of SAS similarity criterion: If two triangles are similar, then one of the angles of one triangle is equal to the corresponding angle of the other triangle and the sides including these angles are in constant proportion.

12. Pythagoras Theorem: In a right triangle, the square of the hypotenuse is equal to the sum of the squares of the other two sides. 

13. Converse of Pythagoras Theorem: If in a triangle, square of one side is equal to the sum of the squares of the other two sides, then the angle opposite the first side is a right angle.

Top Concepts

1. All congruent figures are similar but the similar figures need not be congruent.

2. Two polygons are similar if

* Their corresponding angles are equal

* Their corresponding sides are in same ratio.

3. If the angles in two triangles are:

* Different, the triangles are neither similar nor congruent.

* Same, the triangles are similar.

* Same and the corresponding sides are the same size, the triangles are congruent

4. A line segment drawn through the mid points of one side of a triangle parallel to another side bisects the third side

5. The ratio of any two corresponding sides in two equiangular triangles is always same.

6. All circles are similar.

7. All squares are similar.

8. All equilateral triangles are similar.

9. If two triangles ABC and PQR are similar under the corresponding A ↔ P, B ↔Q and C ↔ R, then symbolically, it is expressed as Δ ABC  Δ PQR.

10. If two angles of a triangle are respectively equal to two angles of another triangle, then by the angle sum property of a triangle their third angles will also be equal.

11. The ratio of the areas of two similar triangles is equal to the square of the ratio of their corresponding sides.

12. The ratio of the areas of two similar triangles is equal to the ratio of the squares of the corresponding medians.

13. Triangles on the same base and between the same parallel lines have equal area.

14. In a rhombus sum of the squares of the sides is equal to the sum of squares of the diagonals.

15. In an equilateral or an isosceles triangle, the altitude divides the base into two equal parts.

16. The altitude of an equilateral triangle with side ‘a’ is √3/2 a.

17. In a square and rhombus, the diagonals bisect each other at right angles

18. If a perpendicular is drawn from the vertex of the right triangle to the hypotenuse then triangles on both sides of the perpendicular are similarto the whole triangle and to each other.

Top Formulae

Top Diagrams

1. Δ ABC ∼ Δ DEF

2. Δ ABD ≅ Δ DEF

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