∵ cos(α+β)=1/5,cos(α-β)=3/5 α,β为锐角
∴ sin(α+β)=2√6/5,sin(α-β)=4/5 (不妨设α>β)
sin2α=sin((α+β)+(α-β))=sin(α+β)cos(α-β)+cos(α+β)sin(α-β)
=2√6/5x3/5+1/5x4/5
=(6√6+4)/25
cos2α=cos((α+β)+(α-β))=cos(α+β)cos(α-β)-sin(α+β)sin(α-β)
=1/5x3/5-2√6/5x4/5
=(3-8√6)/25
∵ cos(α+β)=1/5,cos(α-β)=3/5 α,β为锐角
∴ sin(α+β)=2√6/5,sin(α-β)=4/5 (不妨设α>β)
sin2α=sin((α+β)+(α-β))=sin(α+β)cos(α-β)+cos(α+β)sin(α-β)
=2√6/5x3/5+1/5x4/5
=(6√6+4)/25
cos2α=cos((α+β)+(α-β))=cos(α+β)cos(α-β)-sin(α+β)sin(α-β)
=1/5x3/5-2√6/5x4/5
=(3-8√6)/25