f'(x)=1/2x^(-1/2)
由拉格朗日中值定理得
f(4)-f(1)=(4-1)f'(ξ)
f'(ξ)=(2-1)/3=1/3=1/2ξ^(-1/2)
=> ξ=(2/3)^(-2)=9/4