sinx＋cosx
=√2[(√2/2)sinx＋(√2/2)cosx]
=√2[sinxcos(π/4)＋cosxsin(π/4)] 【利用公式sin(a＋b)=sinacosb＋cosasinb的逆用】
=√2sin(x＋π/4)