方法一:
y= asinx+bcosx=√(a^2+b^2)sin(x+arctanb/a)
所以y=sinx+cosx=√2sin(x+pi/4)
方法二:
sinx+cosx
=√2(cos45°sinx+sin45°cosx)
=√2sin(x+45°)
==√2sin(x+π/4)