If the function f (x) is differentiable at x = a, then Lim h → 0 (f (a + 3H) - f (A-H)) △ 2H =?
=lim {f(a+3h)--f(a)+f(a)--f(a--h)}/2h
=lim 3/2*[f(a+3h)--f(a)]/(3h)+lim 1/2*[f(a--h)--f(a)/(--h)]
=3/2*f'(a)+1/2*f'(a)
=2f'(a)
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