**Important Concepts and Formulas**

1. If two objects have the same shape
and the same size, they are called congruent objects.

2. If Î”ABC ≅ Î”PQR then ∠A = ∠P, ∠B = ∠Q, ∠C = ∠R and
AB = PQ, BC = QR and AC = PR.

4. If the three sides of one triangle
are equal to the corresponding three sides of the other triangle, then the two
triangles are congruent. This condition is called SSS congruence of two
triangles.

5. If two sides and the angle included
between them in one triangle are equal to the corresponding sides and the angle
included between them in the other triangle, then the given triangles are
congruent. This condition is called SAS congruence of triangles.

6. If two angles and the side included
between them in one triangle are equal to the corresponding angles and the side
included between them in the other triangle, then the given triangles are
congruent. This is called ASA condition of congruence.

7. If the hypotenuse and one side of one
right-angled triangle are equal to the hypotenuse and the corresponding side of
another right-angled triangle, then the two right-angled triangles are
congruent. This is called RHS congruence of right-angled triangles.