y=sinx+cosx
=√2(√2/2*sinx+√2/2cosx)
=√2(sinxcosπ/4+cosxsinπ/4)
=√2sin(x+π/4)
对称轴sinsin是最值
所以sin(x+π/4)=±1
x+π/4=kπ+π/2
x=kπ+π/4有选项A.x=45度 B.x=120度 c.x=130度D.x=120度应该x=－π/4