方程组3x y=30 y-2x=5 解方程

方程组3（x²+y²）-5xy=6(1)2（x²+y²）+xy=17(2)(1)*2-(2)*3-13xy=-39xy=3(1)+(2)*513（x²

y=3x^2-8,代入第2个方程得：x^2+x(3x^2-8)+3x^2-8=43x^3+4x^2-8x-12=0解得只有一个实根：x=1.67758093466546故y=0.442833377再问

xy/(2x+y)-xy/(2x-y)=3xy/(2x-y)+xy/(2x+y)=4；一、两公式对加得2xy/(2x+y)=7xy=(2x+y)*7/2二、把xy代入原公式求解就行了：(2x+y)*7

x-y=a+3(1)2x+y=5a(2)(1)+(2)得3x=6a+3x=2a+1y=a-2x>y>0a-2>0a>22a+1>a-2a>-3综上a>2a取不超过4的正整数,a≤4a>2a=3或a=4

∵2xy-5√(xy+1)=10==>2xy+2-5√(xy+1)=12==>2(xy+1)-5√(xy+1)-12=0==>[2√(xy+1)+3][√(xy+1)-4]=0(用十字相乘法分解)==

x²-2xy+y²=12x²-5xy-3y²=0即(x-y)²=1(2x+y)(x-3y)=02x+y=0或x-3y=0如果2x+y=0,则y=-2x

xy=-a2+a+2=(2-a)(a+1)=[-(2-a)][-(a+1)]则x=2-ay=a+1x+y=3或者x=-(2-a)y=-(a+1)x+y=-3又因为x+y=3所以x=2-ay=a+1

再问：2y=3x是怎么来的再答：比例的性质，内项积等于外项积

x的平方+xy=2①y(x+y)=3②两式相加得x平方+y平方+2xy=5则(x+y)平方=5则x+y=√5或-√5当x+y=√5,代入①,解得x=2/√5,代入②,解得y=3/√5当x+y=-√5,

3(x²+y²）-5xy=6；①2(x²+y²)+xy=17②①+②×5,得3(x²+y²)+10(x²+y²)=6+8

两式相加得x=4+xy即x=4/(1-y)①两式相减得y=-1-4xy②①代入②y=-1-4y*4/(1-y)整理得y²-16y-1=0解得y=8±√65代入①,即可算出x了如有不明白,可以

1、x-y=a+32x+y=5a二式相加得3x=6a+3,得x=2a+1代入得y=a-2所以方程组解为x=2a+1,y=a-22、2a+1＞a-2,解得a＞-3a-2＞0,解得a＞2综上可得a＞23、

x+y=-3x=-3-y带入xy=2(-3-y)y=2-y^2-3y-2=0y^2+3y+2=0(y+1)(y+2)=0y1=-1y2=-2x1=-2x2=-1

由(x+y)/xy=5/6(y+z)/yz=-2/3得1/x+1/y=5/6①1/y+1/z=-2/3②1/x-1/z=3/21/z=1/x-3/2③z=2x/(2-3x)代入2(x+3)+xz=0化

2x²-3xy-5y²=7即(2x-5y)(x+y)=7而2x-5y=1所以代入得到x+y=7于是x=7-y代入2x-5y=114-2y-5y=1那么7y=13解得y=13/7而x

x=2,y=0.5

2x^2-3xy-5y^2=7即(x+y)(2x-5y)=72x-5y=1所以x+y=7x=36/7,y=13/7

由(x-2)(y+3)=xy(x-5)(y+2)=xy得3x-2y-6=02x-5y-10=0上面方程化为6x-4y-12=06x-15y-30=0两式相减得11y=-18,y=-18/11,代入任意

x^2-xy+y^2=3,2x^2-xy-y^2=55x^2-5xy+5y^2=156x^2-3xy-3y^2=15两式子相减x^2+2xy-8y^2=0(x+4y)(x-2y)=0x=2y或者x=-

4x-3y=k①2x+3y=5②①-②×2得:4x-3y-4x-6y=k-10-9y=k-10y=-(k-10)/9①+②得:4x-3y+2x+3y=k+56x=k+5x=(k+5)/6因为x