a=8x,y互为相反数即X+Y=0 X=-Y3X-5Y=2A 3X+5X=2A 8X=2A X=0.25A Y=-0.25A2X+7Y=A-18 2*0.25A+7*(-0.25A)=A-18 即A=8
3x-5y=2a2x+7y=a-18解得为31x=19a-9031y=-a-18x+y=019a-90-a-18=018a=108a=6
