6. (f(x)在 _{0)=(x)_(0) 处可导,且 lim _(harrow 0)dfrac (f({x)_(0)+2h)-f((x)_(0)-h)}(2h)= () .-|||-A. dfrac (3)(2)f((x)_(0)) B -dfrac (2)(3)f((x)_(0)) C. dfrac (3)(2)f'((x)_(0)) D. -dfrac (3)(2)f'((x)_(0))-|||-7.设 (x)=arctan (e)^x, 则 '= ()