(1) (x+y)^2 = x^2+xy + xy+y^2=12+13=25
(2) y^2-x^2=1,
若x+y =5 则y-x=0.2 则x=2.6,y=2.4 则 x^2+y^2=12.52
若x+y=-5 则y-x=-0.2 则x=-2.4 y=-2.6 则x^2+y^2=12.52
总之,x^2+y^2=12.52
(3) x^4+y^4=78.8752（3）是怎么得来的啊。。。