Here, we will determine the ARC LENGTH to make a 90 degree angle with a Radius of 36 inches.
On your scientific calculator punch in 36 x ∏(3.14159) = you get 113.1.
Divide this by 180. You get .628. Now multiply this by 90. You should get 56.55 inches.
The ARC LENGTH at 90 degrees with a 36 inch radius is 56.55 inches long.
In short:
Radius x ∏= ____ ÷ 180 ____ x Angle = ARC LENGTH.
This exercise is good for all angles with less than 180 degrees of arc.