Q:

Approximating smooth function on $[0,1]$ by Bernstein polynomial (interested in approximation rate in $L^2$ norm)

Consider a smooth function $f$ on $[0,1]$ and its Bernstein polynomial of power $n$: 
$$B_n(f)=\sum_{k=0}^n f\left(\frac{k}{n}\right) b_{n,k}(x)$$
with
$$b_{n,k}(x) = \binom{n}{k}x^k (1-x)^{n-k}.$$
It is well known that 
$$\sup_{x \in[0,1]} |B_n(f)(x)-f(x)|=O\left(\frac{1}{\sqrt{n}}\right).$$
But what if one considers the $L^2$ norm instead of $L^{\infty}$ norm? Will the approximation power be better in $L^2$ norm? In particular, is it possible to conclude that 
$$\left(\int_{0}^1 (B_n(f)(x)-f(x))^2 dx \right)^{\frac{1}{2}}=O\left(\frac{1}{n}\right)?$$

A:

Page 206 of the book Bernstein Operators and Their Properties states for $f\in C^2[0,1]$:
$$B_n(f,x)-f(x)=\frac{x(1-x)}{2n}f''(x)+o\left(\frac{1}{n}\right)$$
and cites 

E.V. Voronovskaya, The asymptotic behavior of the approximation of functions by their
  Bernstein polynomials. Doklady SSSR 4, 79–85 (1932) (in Russian)

