已知cosa=1/7,cos(a-b)=13/14,且0＜b＜a＜派/2.求b
问题描述：
已知cosa=1/7,cos(a-b)=13/14,且0＜b＜a＜派/2.求b
答
0＜b＜a＜π/20＜a-b＜π/2cosa=1/7,sina=√(1-(1/7)^2)=4√3/7cos(a-b)=13/14,sin(a0b)=√(1-(13/14)^2)=3√3/14sinb=sin[a-(a-b)] = sinacos(a-b)-cosasin(a-b)= 4√3/7*13/14-1/7*3√3/14= (52√3-3√3)/98= √3/...sin(a0b)=√(1-(13/14)^2)=3√3/14这一步是？sin(a-b)=√(1-(13/14)^2)=3√3/14