Question

Why does the congruency of all angles not prove the congruency of two triangles?

Answer 1

Because even though the angles may be the same, the lengths can be different. In an isosceles triangles, this may be the case.

Explanation:

Answer 2

Draw an equilateral triangle with side lengths 5 inches each. Each interior angle is 60 degrees (this is true of any equilateral triangle).

Now draw an equilateral triangle of 10 inches each. The angles will be the same as before. We can see that the triangles are not congruent. Congruent triangles must have the same side lengths, but clearly the second one is larger than the first.

This is an example of why knowing solely the congruency of the angles is not enough to prove the triangles congruent or not. We would need to know something about the sides (whether they are congruent or not) to be able to determine overall triangle congruency.