[질문] 수2삼각함수공식중에sinxsiny,cosxcosy,sinxcosy,cosxsiny를합공식으로좀알려줘요

sinxcosy=1/2{sim(x+y)+sin(x-y)},cosxsiny=1/2{sim(x+y)-sin(x-y)}, cosxcosy=1/2{cos(x+y)+cos(x-y)},sinxsiny=1/2{cos(x+y)-cos(x-y)}