sina+cosa
=√2(sina*√2/2+cosa*√2/2)
=√2(sinacosπ/4+cosasinπ/4)
=√2sin(a+π/4)
最小时sin(a+π/4)=-1
所以a+π/4=2kπ-π/2
所以a==2kπ-3π/4