x+y=0.2x+3y=1相加2x+4y=1.2x+2y=0.6原式=3(x²+4xy+4y²)=4(x+2y)²=3×0.6²=1.08
两式相加 x+y+x+3y=0.2+12x+4y=1.2x+2y=0.63x^2+12xy+12y^2＝3（x^2+4xy+4y^2）=3(x+2y)^2=3x0.6^2=3x0.36=1.08
