二元一次方程：{3X=2Y,3X+4Y=36 {3X+4Y=10,4X+Y=9 {2X-Y=6,X+2Y=-2 {X+Y=420,30%X+40%Y=160×80%

{3X=2Y,3X+4Y=36X=2Y/3代入3X+4Y=362Y+4Y=36Y=6X=2*6/3=4 {3X+4Y=10,4X+Y=9X=(10-4Y)/3代入4X+Y=94(10-4Y)/3+Y=940-16Y+3Y=27Y=1X=(10-4*1)/3=2 {2X-Y=6,X+2Y=-2 X=(6+Y)/2代入X+2Y= -2(6+Y)/2+2Y= -26+Y+4Y= -4Y=...