∵cosβ=-1/3,β∈(π/2,π), ∴sinβ=2√2/3∵a+β∈(π/2,3π/2),sin(a+β)=7/9>0 ∴a+β∈(π/2,π),cos(a+β)=-4√2/9∴cosa=cos[(a+β)-β]=cos(a+β)cosβ-sin(a+β)sinβ=(-4√2/9)(-1/3)-(7/9)(2√2/3)=(-10√2)/27
