x、y属于3/4派 到 派,sin(x+y)=-3/5 sin(y-派/4)=12/13 求cos(x+派/4)
问题描述:
x、y属于3/4派 到 派,sin(x+y)=-3/5 sin(y-派/4)=12/13 求cos(x+派/4)
答
x,y∈(3π/4,π)
x+y∈(3π/2,2π)
y-π/4∈(π/2,3π/4)
cos(x+y)>0
cos(y-π/4)