设f(x)在x=a处可导,f'(x)=b 求极限lim(h-0) f(a-h)-f(a+2h)/ h

lim [h→0] [f(a-h)-f(a+2h)]/h=lim [h→0] [f(a-h)-f(a)+f(a)-f(a+2h)]/h=lim [h→0] [f(a-h)-f(a)]/h + lim [h→0] [f(a)-f(a+2h)]/h=-lim [h→0] [f(a-h)-f(a)]/(-h) - 2lim [h→0] [f(a+2h)-f(a)]/(2h)=-f '(a)-...