x+y=6(x+y)^2=36x^2+y^2+2xy=362xy=36-x^2-y^22xy=24xy=12
x+y=6平方x²+y²+2xy=362xy=36-(x²+y²)=36-12=24xy=12
（x+y）^2=x²+y²+2xy=36 所以xy=12
x²+y²=12，即(x+y)²-2xy=12,将第二个式子代入，即6²-2xy=12，计算出xy=12
