求Z=x^2 2y^2和z=6-2x^2-y^2

X+Y-Z=6①Y+Z-X=2②Z+X-y=0③①+②+③得x+y+z=8④④-①得2z=2z=1④-②得2x=6x=3④-③得2y=8y=4即x=3y=4z=1

实数x,y,z,满足那么x+y=6,z^2=xy-9,∴xy=z^+9,(x-y)^=(x+y)^-4xy=-4z^>=0,∴z=0,(x+y)^z=6^0=1.

2x+6y+z=73x+9y+z=8两式相减x+3y=1x=1-3y两式相加5x+15y+2z=155-15y+15y+2z=15z=5x+y+z=1-3y+y+5=6-2y得不到x+y+z=5的结论

(5x+3y+2z)+(4x+6y+7z)=2011+20129(x+y+z)=4023x+y+z=447

x+y-z=6y+z-x=2z+x-y=0三式相加得x+y+z=8-得2z=2z=1-得2x=6x=3-得2y=8y=4x=3y=4z=1

x=z(lnz-lny)=zlnz-zlny令F(x,y,z)=zlnz-zlny-xaF/ax=-1aF/ay=-z/yaF/az=lnz+1-lny所以az/ax=-Fx/Fz=1/(lnz+1-

设：x/4=y/5=z/6=k则有：x=4k,y=5k,z=6k(x+y+z)/(3x-2y+z)=(4k+5k+6k)/(12k-10k+6k)=15k/8k=15/8

2x-3y-4z=01式x+y+z=02式1式+2式×4得到：2x-3y-4z+4x+4y+4z=06x+y=06x=-yx:y=(-1):61式-2式×2得到：2x-3y-4z-2x-2y-2z=0

因为x:y:z=3：4：5所以设x=3k,y=4k,z=5k(k≠0)(1)z/(x+y)=5k/(3k+4k)=5k/7k=5/7(2)x+y+z=63k+4k+5k=612k=6k=1/2x=3k

x+y+z-6=02x+3y-z-12=02x-y-z=0组成方程组再解x=2y=3z=1

/>x/4=y/5=z/6=t分别用t表示x,y,z然后带入到要求的式子x+y+z/3x-2y+z中最终解得结果

这道题看似复杂,但是按步骤解就不难了.对于有绝对值的方程就一定要讨论了.x,y,z都有三种可能,x《-2,-2《x《3,x》3,y《-3,-3《y《1,y>1,z

解法1：2x+5y+4z=0式①3x+y-7z=0式②x+y-z=?式③式①=0,式②=0,所以式①-式③=式②-式③即：2x+5y+4z-x-y+z=3x+y-7z-x-y+zx+4y+5z=2x+

(x+y+z)²+(x-y-z)(x-y+z)-2·z(x+y)=(x+y)²+2z(x+y)+z²+(x-y)²-z²-2z(x+y)=(x+y)&

0和－2再答：0和－2再问：说清是X等于几和z等干几

/>x+y-z=6①x-3y+2z=0②3x+2y-z=0③由③-①得：2x+y=-6④由②+③×2得7x+y=0⑤由⑤-④得：5x=6x=6/5把x=6/5代入⑤得：42/5+y=0y=-42/5把

2x+y-3z=1,①x-2y+z=6,②3x-y+2z=9③①+③得：5x-z=10④①×2+②得：5x-5z=8⑤④-⑤得：4z=2∴z=1/2x=21/10=2.1y=-1.7

y=1+z,x=4-z,则x^2+y^2+z^2=3(z-1)^2+14,所以min=14再问：详细点，∈是什么意思再答：包含于，也就是属于的意思，即x,y,z都是实数，R是实数的意思吧再问：过程，y

X+Y-Z=6.aY+Z-X=2.bZ+X-Y=0.ca,b,c三式相加X+Y+Z=8.dd式-a式2Z=2Z=1d式-b式2X=6X=3d式-C式2Y=8Y=4

括号是什么意思?只有一半