4x+8x+8y=4y+9y+9x
4x+8(x+y)=4y+9(x+y)
4x=4y+(x+y)
4x=4y+840
x=y+210
代入x+y=840
y+210+y=840
2y=630
y=315
x=y+210=525
{x+y=840;4x+8x+8y=4y+9y+9x
4x+8x+8y=4y+9y+9x
4x+8(x+y)=4y+9(x+y)
4x=4y+(x+y)
4x=4y+840
x=y+210
代入x+y=840
y+210+y=840
2y=630
y=315
x=y+210=525