Mark M. answered 05/09/20
Retired math prof. Calc 1, 2 and AP Calculus tutoring experience.
The domain of y = √(64 - x2) is [-8,8].
Average value = [1 / (8-(-8)] ∫(-8 to 8)√(64 - x2) dx
= (1/16) ∫(-8 to 8 ) √(64 - x2)dx
The graph of y = √(64 - x2) is the upper half of the semicircle centered at (0,0) with radius 8.
The value of the integral is then the area of this semicircle.
So, average value = (1/16) [(1/2)π(8)2] = 2π