find the average rate of change of f from pi to 11pi/3. f(x) = cos(x/2) ...?

the average rate of f is [f(x2)  - f(x1)] / (x2  - x1) x1  =pi, and x2 = 11pi/3 f(x2) = f(11pi/3) = cos [11pi/6)] f(x1) = f(pi) = cos [pi/2)]=0 so  M=  f(x2) -f(x1)  / 11pi/3  - pi = cos [11pi/6)]  /  8pi/3 finally M= 3cos [11pi/6)]  /  8pi