Question: how to find the perimeter of a 90 degree triangle?
Answer: The perimeter of a shape is the length around the outside of that shape. ... 
A triangle has three sides, a, b, and c. ... 
P = a + b + c.
P = 3 + 4 + 5 P = 12.
The Pythagorean Theorem is a formula that shows the relationship between the lengths of a right triangle's sides.
a^2 + b^2 = c^2.