Find (?)

Let fbe a function. Consider the functionA(t)=1/t(upper=t,lower=0) f(x)dx,which gives the average off on the interval[0,t], fort>0.

Suppose that Ahas a local maximum att=b, thenA'(b)=0.

Now, the formula for A'(t)=(?)-(1/t^2)(upper=t,lower=0) f(x)dx=1/t((?)-A(t))