设x^2+y^2-xy＝t (1)x^2+y^2+xy＝1 (2)由(1)(2)可解得：x^2+y^2=(t+1)/2 (3)2xy=1-t (4)(3)+(4)化简得：(x+y)^2=(3-t)/2(3)-(4)化简得：(x-y)^2=(3t-1)/2因为：(x+y)^2≥0,(x-y)^2≥0所以(3-t)/2≥0,(3t-1)/2≥0解得...