已知x y=0.2,x+3y=1,求3x的二次方

1/x+1/y=3∴x+y=3xy3x-2xy+3y/x+xy+y=[3(x+y)-2xy]/[(x+y)+xy]=(9xy-2xy)/(3xy+xy)=7/4

代入方法：分母：3x-3xy+3y=3(x+y)-3xy=3•(3xy)-3xy=9xy-3xy=6xy分子：x+xy+y=x+y+xy=3xy+xy=4xy所以原式=6xy/4xy=3/

已知1/x-1/y=3,应该是1/x+1/y=3,[按原式得不出结果!]如此,则3xy＝x+y（3x-xy+3y）/（x+xy+y）＝2

x+y=3,xy=14x+3xy+4y=4(x+y)+3xy=4x3+3x1=12+3=15祝学习进步,望采纳,不懂的欢迎追问.再问：怎么又是你啊。。。再问一道题：代数式x²＋x＋3的值为7

=-x-(2y-2+3x)+2(x+4)=-x-2y+2-3x+2x+8=-4x-2y+10

绝对值大于等于0,相加等于0,若有一个大于0,则另一个小于0,不成立所以两个都等于0所以x+y+1=0xy+3=0xy=-3x+y=-1两边平方x^2+2xy+y^2=(-1)^2x^2+y^2=1-

结果为：7,对吧.1.把“1/x+1/y=2”代换为y+x=2xy具体方法:将等式同时乘以“xy”即(xy)*1/x+（xy)*1/y=2xy化简后为：y+x=2xy.a2.化简“（3x+xy+3y)

(1)已知x+y=6,xy=-3x^2y+xy^2=xy(x+y)=-3*6=-18(2)x*[2-(1/x)]+(x/x^2-2x)*(x^2-4)=x*(2x-1)/x+[x/x(x-2)]*(x

(-2xy+2x+3y)-(3xy+2y-2x)-(x+4y+xy)=-6xy+3x-3y=-6×(-2)+3×1=15

5xy+4x+7y+6x-3xy-4xy+3y=5xy-3xy-4xy+4x+6x+7y+3y=（5-3-4）xy+（4+6）x+（7+3）y=-2xy+10x+10y=-2xy+10（x+y）因为x

/>∵1/x=1/y=6∴x=y=1/6∴(3x-2xy+3y)/(3xy+x+y)=(3/6-2*1/6*1/6+3/6)/(3*1/6*1/6+1/6+1/6)上下同乘36=(18-2+18)/(

是求(3x+xy+3y)/(x-xy+y)吧?(3x+xy+3y)/(x-xy+y)上下同除以xy=(3/y+1+3/x）/(1/y-1+1/x)=[3(1/x+1/y)+1]/(1/x+1/y-1)

（1）∵xy+x=-1①，xy-y=-2②，∴①-②得x+y=1；（2）先把xy+x=-1，xy-y=-2的值代入代数式，得原式=-x-[2y-1+3x]+2[x+4]=-x-2y+1-3x+2x+8

3/(x-y)=1/xyx-y=3xyy-z=-3xy原式=[(y-x)-2xy]/[2(x-y)+3xy]=[(-3xy)-2xy]/[2(3xy)+3xy]=-5xy/9xy=-5/9

x^3y-2x^2y^2+xy^3=xy(3x^2-4xy+y^2)=xy(3x-y)(x-y)x-y=-1,xy=3y(y-1)=3

∵x-y=4xy，∴2x+3xy-2yx-2xy-y=2(x-y)+3xyx-y-2xy=8xy+3xy4xy-2xy=112．故答案为：112．

xy+1/xy>=2√（xy*1/xy）=2(当xy=1/xy即xy=1时取等号)x/y+y/x>=2√（x/y*y/x）=2(当x/y=y/x即x=y取等号)当x=y=1时可以同时满足两项的等号要求

因为xy/（x+y）=1/2所以x+y=2xy原式=3（x+y)-5xy/(-x-y+3xy)=3*2xy-5xy/(-2xy+3xy)=xy/xy=1

（-2xy+2x+3y)-(3xy+2y-2x)-(x+4y+xy)=(-2xy-3xy-xy)+(2x+2x-x)+(3y-2y-4y)=-6xy+3x-3y=-6+3*3=3

(-2xy+2x+3y)-(3xy+2y-2x)-(x+4y+xy)=-2xy+2x+3y-3xy-2y+2x-x-4y-xy=-6xy+3x-3y=-6*(-2)+3*1=15不懂可追问,有帮助请采