Because this is a right triangle, we can use the Pythagorean theorem:
a^2 + b^2 = c^2
First, replace each letter with its given variable:
8^2 + 20^2 = c^2
Simplify:
64 + 400 = c^2
Finally, you have to find the square root of the result:
c = 21.5
ST = 21.5 inches
