To prove a triangle has at most one right angle by using indirect proof, the assumption to be made is: Blank 1 Question 30 .

The assumption to be made is: A triangle can have more than one right angle.

Step 1: Assume the contrary of what you want to prove. In this case, assume that a triangle can have more than one right angle.

Step 2: Try to deduce a result from this assumption that contradicts a known fact. If a triangle has two right angles, then the sum of the angles of the triangle would be more than 180 degrees (90+90+some positive number).

Step 3: However, we know that the sum of the angles in a triangle is always 180 degrees. This is a contradiction.

Step 4: Therefore, the assumption that a triangle can have more than one right angle must be false. Hence, a triangle has at most one right angle.