设 x =cos a,y =sin a,则 x +y =1/5,(1)x^2 +y^2 =1.(2)(1)^2 -(2) 得2 xy = -24/25,即 xy = -12/25.(3)由(1)(3) 可知,x,y 是方程z^2 -(1/5)z -12/25 =0.(4)的两根.解(4)得z1 =4/5,z2 = -3/5.所以 sin a =4/5,cos a ...