1、原式=sin(a＋b)cos(y－b)＋cos(a＋b)sin(y－b)
=sin[(a＋b)＋(y－b)]
=sin(a＋y)
2、原式=－[cos(a－b)cos(b－y)－sin(a－b)sin(b－y)]
=－cos[(a－b)＋(b－y)]
=－cos(a－y)