（x-y)^2=625/36
x-y=±25/6
（x+y）²=49/36
(x+y)²-（x-y）²=4xy=49/36-625/36=-576/36=-16
xy=-4
x^2-y^2=(x+y)(x-y)=±175/36
x^2+y^2=(x+y)²-2xy=49/36-2×(-144/36)=337/36