f'(x)=cosx-sinx
g(x)=f(x).f'(x)=(sinx+cosx)(cosx-sinx)=(cosx)^2-(sinx)^2=cos(2x)
當(dāng)2x=2kπ+π (k∈Z)時(shí),g(x)有最小值g(x)min=-1,此時(shí)
x=kπ+π/2 (k∈Z),集合為{x|x=kπ+π/2 k∈Z}