Question

how can we prove that there are 3 corners in a triangle

Answer 1

It is in its name itself..

Tri - Three
Angle - Angle

There are three angles. So there must be 3 corners.

Hope this helps :)

Answer 2

this is already a proven fact...... as tri means 3 and angle yu well knows....... so combination of 3 different angle lines meet at 6 points out of which the 3 coincides at the mid of line and rest 3 forms the 3 corners