limh→0[f(x-2h)-f(x)]/3h=?

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limh→0[f(x-2h)-f(x)]/3h=?

lim[f(x-2h)-f(x)]/3h
=lim[f(x-2h)-f(x)]/(-2h)·(-2/3)
=-2/3·lim[f(x-2h)-f(x)]/(-2h)
= -2/3·f '(x)