∵0∴π/4＜α+π/4＜3π/4 π/4＜π/4-b/2＜π/2sin(π/4+α)=2√2/3 sin(π/4-b/2)=√6/3∴cos(a+b/2)=cos((α+π/4)-(π/4-b/2))=cos(α+π/4)cos(π/4-b/2)+sin(α+π/4)sin(π/4-b/2)=1/3×√3/3+2√2/3×√6/3=5√3/9
