x+3y+xy+9+求x+3y最小值

须饰袁5198求x - xy+y分之3x+xy+3y 其中xy分之x+y=3 -
关秆肢14765384351 ______ xy分之x+y=3 x+y=3xy x-xy+y分之3x+xy+3y =[3(x+y)+xy]/[(x+y)-xy]=(9xy+xy)/(3xy-xy)=10xy/2xy=5

须饰袁5198丨x - y - 1丨十(xy十2)的平方=o,求( - 2xy十2x十3y)一(3xy十2y一2x)一(x -
关秆肢14765384351 ______ 解:由题意可得:丨x-y-1丨+(x+y+2)^2=0 x-y-1=0,x+y+2=0 联立解得:x=-1/2,y=-3/2 原式=(-2xy十2x十3y)一(3xy十2y一2x)一(x+4y+xy)=-2xy+2x+3y-3xy-2y+2x-x-4y-xy=(-2-3-1)xy+(2+2-1)x+(3-2-4)y=-6xy+3x-3y=-9/2-3/2+9/2=-3/2

须饰袁5198x - y=3 x^2+y^2=13 求x^3 - 8x^2y^2+xy^3 -
关秆肢14765384351 ______ 正确应该是求:x^3y-8x^2y^2+xy^3 由x-y=3 得:(x-y)^2=9 x^2-2xy+y^2=913-2xy=9 xy=2 所以:x^3y-8x^2y^2+xy^3=x^3y+xy^3-8x^2y^2=xy(x^2+y^2)-8(xy)^2=2*13-8*4=26-32=-6

须饰袁5198当实数x,y满足xy+x+y+7=0,3x+3y=9+2xy,则x平方y+xy平方=多少 -
关秆肢14765384351 ______ x^2y+xy^2=xy*(x+y)因为x+y=-(7+xy) 又x+y=(9+2xy)\3所以(9+2xy)/3=-(7+xy) 3+2xy/3=-7-xy 5xy/3=-10 解得 xy=-6所以x+y=-(7+xy)=-1所以x^2y+xy^2=xy*(x+y)=-6*(-1)=6

须饰袁5198已知x+y+z=5,xy+yz=9,求x的平方+y的平方+z的平方的值. -
关秆肢14765384351 ______[答案] x+y+z=5,xy+yz=9(应该是xy+xz+yz吧?) x+y+z=5 (x+y+z)^2=25 x^2+y^2+z^2+2xy+2xz+2yz=25 x^2+y^2+z^2+2(xy+xz+yz)=25 x^2+y^2+z^2+2*9=25 x^2+y^2+z^2=25-18 x^2+y^2+z^2=7

须饰袁5198初一数学题:已知xy/x+y=3,求代数式3x - 5xy+3y/ - x+3xy - y的值. -
关秆肢14765384351 ______ ∵xy/x y=3∴xy=3(x y) 原式=- 3x-5xy 3y/x-3xy y =- 3(x y)-5xy/(x y)-3xy =- 3(x y)-15(x y)/(x y)-9(x y) =- 12(x y)/8(x y)【算出来上下都为负,则约掉】 =-3/2 希望楼主采纳答案

须饰袁5198已知X+Y=3,X^2+Y^2+3XY=4,求X^3Y+XY^3的值 -
关秆肢14765384351 ______[答案] x^2+y^2+3xy=4 (1) 因为x+y=3,得(x+y)^2=x^2+y^2+2xy=9 即x^2+y^2+2xy=9 (2) (1)-(2)得 xy=-5 由(1)知, x^2+y^2=4-3xy 所以x^3y+xy^3=xy(x^2+y^2) =xy(4-3xy) =(-5)[4-3*(-5)] =95

须饰袁5198若2lg(x - 3y)=lgx+lg(4y)求x/y的值 -
关秆肢14765384351 ______ lg(x-3y)^2=lg(x*4y) (x-3y)^2=4xy x^2+9y^2-6xy-4xy=0 (x/y)^2-10(x/y)+9=0 (x/y-1)(x/y-9)=0 x/y=1 or x/y=9

须饰袁5198已知X3+Y3=—27,X2—XY+Y2=9,求X+Y的值 -
关秆肢14765384351 ______[答案] 这个题目应该是用试验法来做. x3+y3=(x+y)(x2-xy+y2)=(x+y)[(x+y)2-3xy] 因为(x+y)2>=4xy{x+y为正值} 所以x+y在这里必为负值,不然x3+y3就不能为负了. 所以可以设x+y=-3 进一步设x=1,则y=-4,(x+y)2-3xy=6,不合 2 -5 24,此值越来越大,不合,此...

须饰袁5198x^2 - xy=4y^2 - xy=9求2x^2+xy减3y^2 -
关秆肢14765384351 ______ 解:x²-xy=4 ① y²-xy=9 ② ①*2-②*32(x²-xy)-3(y²-xy)=4·2-9·32x²-2xy-3y²+3xy=-192x²+xy-3y²=-192x²+xy-3y²的值为-19