[질문] 삼각함수 공식중 곱을합으로바꾸는공식좀요ㅜ

sin(a)cos(b)=1/2{sin(a+b)+sin(a-b)}/cos(a)sin(b)=1/2{sin(a+b)-sin(a-b)}/cos(a)cos(b)=1/2{cos(a+b)+cos(a-b)}/sin(a)sin(b)=-1/2{cos(a+b)-cos(a-b)}입니다