设 $f\in C[0,1]$ 适合 $$\bex \int_x^1 f(t)\rd t\geq \frac{1-x^2}{2},\quad \forall\ x\in [0,1]. \eex$$ 试证: $$\bex \int_0^1 f^2(t)\rd t\geq \frac{1}{3}. \eex$$