f(x)^g(x)=e^[g(x).lnf(x)]
lim(x→x0):f(x)^g(x)
=e^{lim(x→x0):[g(x).lnf(x)]}
=e^{[lim(x→x0):g(x)][lim(x→x0):lnf(x)]}
=e^[b.ln a]
=a^b
y'=1-acosx
因为0