题目还有点问题
由m*n=1,知2cosθ/(2+sinθ)+2sinθ/(2+cosθ)=1,可化为
2（sinθ+cosθ）=2+sinθ*cosθ
令sinθ+cosθ=t,|t|<=√2
则 代入可得t=1,（t=3舍）
(1)sin(θ+π/4)=√2/2(sinθ+cosθ)=√2/2
(2)sinθ+cosθ=t=1,得θ=2kπ或2kπ+π/2,k∈θ∈[3π/2,-π),
θ=-3π/2
cos(θ+7π/12)=-(√2+√6)/6