sin(a+π/4)=1/3 sina·cos(π/4)+cosa·sin(π/4)=1/3 sina + cosa = √2/3 (1-sina)^(1/2) = √2/3 - sina 2sina -2√2/3·sina -7/9=0 ∵π > a > π/2,sina>0 ∴sina = (4+√2)/6 （另一根(-4+√2)/6被舍去）
