令 x =cos θ,y =sin θ,则 x +y = -√10 /5,(1)x^2 +y^2 =1.(2)(1)^2 -(2),得2xy = -3/5.(3)(2) -(3),得（x -y)^2 = 8/5,所以 x -y = 2√10 /5.(4)或 x -y = -2√10 /5.(5)i) 若 (4) 成立,由 (1)(4) 解得x =√10 /1...