已知锐角x,y满足cosx=3/5 ,cos(x+y)=-5/13 求cosy

问题描述:

已知锐角x,y满足cosx=3/5 ,cos(x+y)=-5/13 求cosy

cosx=3/5→sinx=4/5
cos(x+y)=-5/13
sin(x+y)=12/13
→cos(y)=cos(x+y-x)=cos(x+y)cosx+sin(x+y)siny=-5/13*3/5+12/13*4/5=33/65