设f(x)在[0,1]上连续且可导,又f(0)=0,0≤f'(x)≤1 试证：[ ∫^(0,1)f(x)dx]^2≥∫^(0,1)[f(x)]^3dx
虽然想从0≤f'(x)≤1入手说明f(x)>[f(x)]^3，但貌似没什么用