万能置换
cosθ=[1-tan(θ/2)^2]/[1+tan(θ/2)^2] =4/5
sinθ=[2tan(θ/2)]/[1+tan(θ/2)^2] =-3/5
sin[θ+(π/4)]=sinθ*cos(π/4)+cosθ*sin(π/4)=(√2)/10