(4)设 (x)=(x)^2 leqslant xlt 1, 而 (x)=sum _(n=1)^infty (b)_(n)sin (npi x),-infty lt xlt +infty , 其中 _(n)=2(int )_(0)^1f(x)sin (npi x)dx,-|||-n=1,2,···, 则 (-dfrac (1)(2))= () .-|||-A. -dfrac (1)(8) B. -dfrac (1)(4) C. -dfrac (1)(2) D.1