x+y=30 ,2x+4y=110为什么会等于y=25,x=5

x+y=30 ,2x+4y=110为什么会等于y=25,x=5 为什么(x-2y)(x+4y)-(x-2y)+3(x+4y)-3=(x-2y+3)(x+4y-1). x-y/x-x+y/y-(x+y)(x-y)/y² y/x=2 (x-y) (x-2y)+(x+2y) (x-3y)-2(x-3y) (x-4y)= (x-4y)(2x+y)= (2x-y)(x+y)-(4x-5y)-6=(2x-y+2)(x+y-3) 为什么这么做,原理是什么? 2x-5y=0,y/x=为什么 4x方y/(x-2y)(x+2y)怎么算[x方/x-2y]-4x方y/(x-2y)(x+2y)=? y'=（y/x）^2+y/x+4微分方程 x*x+2x+y*y-4y+5=0 x= y= 如果|X|=4 |Y|=2 且|X+Y|=X+Y 求X+Y (x+y)^2(x-y)^2(x^4+x^2y^2+y^4)=? {3(x+y)-4(x-y)=4 {x+y/2 + x-y/6=1 3(x+y)+2(x-y)=36 4(x+y)-5(x-y)=2 （x+y）/2- x-y/2=6 3(x+y)=4(x-y) x+y/3+x-y/2=6 4(x+y)-2(x-y)=40 4(x-y)-3(x-y)=9 5(x-y)+2(x-y)=11 3(x+y)-4(x-y)=11,2(x-y)+5(x+y)=27