x/2+y/7=13/14,x+y=?x+y）^2+(x-y)^2=7+13x^2+2xy+y^2+x^2-2xy+y^2=202(x^2+y^2)=20x^2+y^2=10（x+y）^2-(x-y)^2=7-13[(x+y)+(x-y)][(x+y)-(x-y)]=-6(2x)(2y)=-64xy=-6xy=-3/2