Geometry
posted by chica .
Indirect Proofs
Prove a triangle cannot have 2 right angles.
1) Given triangle ABC.
2) Assume angle A and angle B are both right angles is true by one of two possibilities (it is either true or false, so we assume it is true)
This is my first time to do indirect proofs, so if someone can talk me through it so I can do the next problems by myself I would be greatful.

The sum of the angles of a triangle must be 180 degrees. If two angles are 90 degrees, the third angle must be zero.
You need three nonzero angles to make a triangle.
Therefore a triangle cannot have two right angles.
QED