α,β∈（0,π\2）,cos(α+β）=12\13,cos(2α+β)=3\5,则cosβ=

α,β∈（0,π\2）,cos(α+β）=12\13,cos(2α+β)=3\5,则cosβ=
α,β∈（0,π\2）,cos(α+β）=12\13,cos(2α+β)=3\5,则cosβ=

α,β∈（0,π\2）,cos(α+β）=12\13,cos(2α+β)=3\5,则cosβ=
sin(a+b)=5/13
sin(2a+b)=4/5
cosa
=cos[(2a+b)-(a+b)]
=cos(2a+b)cos(a+b)+sin(2a+b)sin(a+b)
=12/13*3/5+5/13*4/5
=56/65