已知角ab sin(a+b)=-3/5 sin(b-π/4)=12/13 求cos(a+π/4))_数学解答

已知角ab sin(a+b)=-3/5 sin(b-π/4)=12/13 求cos(a+π/4))

人气：147 ℃　时间：2020-06-06 18:44:22

解答

由已知可知cos(a+b)=-4/5,cos(b-π/4)=5/13;
则cos(a+π/4)=cos[(a+b)-(b-π/4)]=cos(a+b)cos(b-π/4)+sin(a+b)sin(b-π/4)=-56/65