已知15分之8x=12分7y=1求式子2x 7y等于多少

令(X+Y)/5=(Y+Z)/6=(x+z)/7=k得x+y=5k①y+z=6k②z+x=7k③以上三式左右分别相加得2(x+y+z)=18kx+y+z=9k④④-②得x=3k④-③得y=2k④-①得

[（x的平方+y的平方）-（x-y)的平方+2y(x-y)]/4y=1(x²+y²-x²+2xy-y²+2xy-2y²)/4y=1(4xy-2y

（1）y=x/x(x-1)*(x-1)²/(x-1)(x+1)-2/(x+1)=1/(x+1)-2/(x+1)=-1/(x+1)=1/3∴x+1=-3,x=-4即x=-4时,y=1/3（2）

x²-7xy+12y²=0(x-4y)(x-3y)=0x=4y或x=3y当x=4y时；（x-y）分之（x+y）=(5y)/(3y)=5/3当x=3y时；（x-y）分之（x+y）=(

34-51y-60+5y=8+56y102y=34y=1/312-2y-10+y=1y=18x-4x/3+1=3x11x/3=-1x=-3/11

X+Y=7XY=12解得x=3,y=41/x-1/y=1/3-1/4=1/12再问：怎么解得再答：X+Y=7y=7-x带入XY=12x(7-x)=12x^2-7x+12=0(x-3)(x-4)=0x=

已知分式4x^2-1分之6x^2+x-2的值为0,则x^-1=∵分式4x^2-1分之6x^2+x-2的值为0∴6x²+x-2=04x²-1≠0(3x+2)(2x-1)=0∴3x+2

∵（2x+y)/2=(5x+2y)/4=1∴{2x+y=2(1){5x+2y=4(2)(1)==>y=2-2x代入（2）5x+2(2-2x)=4x+4=4,∴x=0,y=2∴(x+2y+1)/(2x-

7y的平方+（18-13x的2次方-5y的平方）-4分之3（12-4x的平方+3分之8y的平方）=7y^2+18-13x^2-5y^2-9+3x^2-2y^2=-10x^2+9x^2=36与y值无关原

x+y+z=0x+y=-zx+z=-yy+z=-x(x+y+z)^3=0x^3+y^3+z^3+3x^2y+3x^2z+3y^2x+3y^2z+3z^2x+3z^2y+6xyz=0x^3+y^3+z^

∵X-8≥0,8-X≥0,∴X=8,∴y=0+0+18=18,∴(x+y)/(√x-√y)-2xy/(x√y-y√x)=(x+y)/(√x-√y)-2xy/[(√(xy)(√x-√y)]=(x+y)/

8x/15=7y/12=1,所以x=15/8,y=12/7那么2x+7y=2×(15/8)+7×(12/7)=(15/4)+12=3又4分之3+12=15又4分之3

需要算算,等我哈,

你确定是2x+7x吗再问：是2x+7y再问：要过程再答：嗯好再答： 

由于|x+y|≠x+y,说明x+y0则x+y=-12又4分之3+8又2分之1=-51/4+17/2=-17/4即负4又四分之一2.若y

解(√x/y+√y/x)^2=x/y+y/x+2=(x+y)^2/xy=64/4=16根号y分之x+根号x分之y>0∴根号y分之x+根号x分之y=4

x²-7xy+12y²=0,(x-4y)(x-3y)=0x=4y或x=3yy分之x=4或3.已知x²y²-20xy+x²+y²+81=0,x

1/x-1/y=5两边乘xyy-x=5xyx-y=-5xy所以原式=[6(x-y)+7xy]/[-8(x-y)-9xy]=[6(-5xy)+7xy]/[-8(-5xy)-9xy]=-23xy/(31x

楼上做得不对!x^2+4y^2-2x+8y+5=0x^2-2x+1+4y^2+8y+4=0(x-1)^2+4(y+2)^2=0x=1y=-2原式解得x^2+y^2分之y代入得（负二分之一）如果有问题、

x分之1+y分之1=（x+y)/(xy)=7/12.