若2^x=4^y+3,27^y=3^x-1,求x-y的值有些看不懂啊。

3(x+y)-4(x-y)=11,2(x-y)+5(x+y)=27 (x-y) (x-2y)+(x+2y) (x-3y)-2(x-3y) (x-4y)= 若2/x-1/y=3,求[y/x-y/x-y(x-y/x-x+y)]/x-2y/x的值 若x+y=4,则多项式1/2(x+y)+4(x-y)-3（x-y）-3/2(x+y)-x+y值是（） 若x=-1/4,能否确定代数式(2x-y)(2x+y)+(2x-y)(y-4x)+2y(y-3x)的值? 若X=负四分之一,能否确定代数式(2X-Y)(2X+Y)+(2X-Y)(Y-4X)+2Y(Y-3X)的值? 2（x+y)-3(x-y)=3 ,4(x+y)+3(x-y)=15 x+y/2+x-y/3=6,4(x+y)-3(x-y)=-20 {(x+y)/2+(x-y)/3=6 4(x+y)-3(x-y)=-20 4(x+y)-3(x-y)=-20,2/x+y+3/x-y=6 3(x+y)-2(x-y)=11 4(x+y)+3(x-y)=9 {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 若(x-y)/(x+y)=3 则(x+2y)/(6x-7y)等于 先化简再求值(x-y)(x-2y)+(x-2y)(x-3y)-2(x-3y)(x-4y) x=4 y=5