如下
y=(cosx)^xlny = xlncosx(1/y)y' =lncosx - xtanxy' =[lncosx - xtanx] .(cosx)^x
y = (cosx)^x = e^[xlncosx]y' = e^[xlncosx] (lncosx - xsinx/cosx) = (cosx)^x (lncosx - xsinx/cosx)
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