[微積] 一題可微或不可微的問題
Show whether the function
f(x)= 6x if 0<=x<=8
12x-48 if x>8
can be differentiable at x=8 or not?
若用導數的定義f'(x)=lim [f(x+h)-f(x)]/h
h->0
則f'(8)=lim [f(8+h)-f(8)]/h exist <=> f(x)在x=8時可微
h->0
lim [f(8+h)-f(8)]/h exist 代表左右極限都要存在且相等
h->0
lim [f(8+h)-f(8)]/h = lim [6(8)-6(8)]/h = 0
h->0負 h->0負
lim [f(8+h)-f(8)]/h = lim {[12(8)-48]-6(8)}/h = 0
h->0正 h->0正
左極限=右極限=0 所以此極限存在且=0 所以f(x)在x=8時可微
但此題答案是不可微的
不知道有哪裡錯誤?
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