∴cos(α-β)=.
答案:
8.若sin α=,sin β=,且α,β为锐角,则α+β的值为________.
解析:∵α,β均为锐角,
∴cos α==,cos β==,
∴cos(α+β)=cos αcos β-sin αsin β
=×-×=.
又∵0<α<,0<β<,
∴0<α+β<π,∴α+β=.
答案:
9.已知α、β为锐角,且cos α=,cos(α+β)=-,求cos β的值.
解:∵0<α<,0<β<,∴0<α+β<π.
由cos(α+β)=-,得sin (α+β)=.
又∵cos α=,
∴sin α=.
∴cos β=cos [(α+β)-α]
=cos(α+β)cos α+sin (α+β)sin α
=-×+×=.
10.已知函数f(x)=2sin,x∈R.
(1)求f()的值;
(2)设α,β∈,f=,f(3β+2π)=,求cos(α+β)的值.
解:(1)f=2sin=2sin