Đoạn:

2x
2 + 2y
2 − 3z
2= -100 là như thế nào bạn nhỉ?

Bạn viết lại đề để mọi người hiểu hơn nhé.

\(\dfrac{x}{4}=\dfrac{y}{4}=\dfrac{z}{5}=>\dfrac{2x^2}{32}=\dfrac{2y^2}{32}=\dfrac{3z^2}{75}\)

AD t/c của dãy tỉ số bằng nhâu ta có

\(\dfrac{2x^2}{32}=\dfrac{2y^2}{32}=\dfrac{3z^2}{75}=\dfrac{2x^2+2y^2-3z^2}{32+32-75}=\dfrac{-100}{-11}=\dfrac{100}{11}\)

\(=>\left[{}\begin{matrix}x=\dfrac{400}{11}\\y=\dfrac{400}{11}\\z=\dfrac{500}{11}\end{matrix}\right.\)

Ta có: \(2x^2+xy+2y^2=\dfrac{3}{2}\left(x^2+y^2\right)+\dfrac{1}{2}\left(x^2+2xy+y^2\right)=\dfrac{3}{2}\left(x^2+y^2\right)+\dfrac{1}{2}\left(x+y\right)^2\)

Theo BĐT Bunhacopxky: \(\left(x^2+y^2\right)\left(1+1\right)\ge\left(x+y\right)^2\Rightarrow\dfrac{3}{2}\left(x^2+y^2\right)\ge\dfrac{3}{4}\left(x+y\right)^2\\ \Rightarrow2x^2+xy+2y^2=\dfrac{3}{2}\left(x^2+y^2\right)+\dfrac{1}{2}\left(x+y\right)^2\ge\dfrac{5}{4}\left(x+y\right)^2\\ \Rightarrow\sqrt{2x^2+xy+2y^2}\ge\dfrac{\sqrt{5}}{2}\left(x+y\right)\)

Chứng minh tương tự:

\(\sqrt{2y^2+yz+2z^2}\ge\dfrac{\sqrt{5}}{2}\left(y+z\right)\\ \sqrt{2z^2+xz+2x^2}\ge\dfrac{\sqrt{5}}{2}\left(x+z\right)\)

Cộng vế theo vế, ta được: \(P\ge\sqrt{5}\left(x+y+z\right)=\sqrt{5}\cdot1=\sqrt{5}\)

Dấu "=" \(\Leftrightarrow x=y=z=\dfrac{1}{3}\) 

Bạn tham khảo nhé

https://hoc24.vn/cau-hoi/cho-cac-so-duong-xyz-thoa-man-xyz1cmrcan2x2xy2y2can2y2yz2z2can2z2zx2x2can5.182722154737

\(1,\\ a,A=4x^2\left(-3x^2+1\right)+6x^2\left(2x^2-1\right)+x^2\\ A=-12x^4+4x^2+12x^2-6x^2+x^2=-x^2=-\left(-1\right)^2=-1\\ b,B=x^2\left(-2y^3-2y^2+1\right)-2y^2\left(x^2y+x^2\right)\\ B=-2x^2y^3-2x^2y^2+x^2-2x^2y^3-2x^2y^2\\ B=-4x^2y^3-4x^2y^2+x^2\\ B=-4\left(0,5\right)^2\left(-\dfrac{1}{2}\right)^3-4\left(0,5\right)^2\left(-\dfrac{1}{2}\right)^2+\left(0,5\right)^2\\ B=\dfrac{1}{8}-\dfrac{1}{4}+\dfrac{1}{4}=\dfrac{1}{8}\)

\(2,\\ a,\Leftrightarrow10x-16-12x+15=12x-16+11\\ \Leftrightarrow-14x=-4\\ \Leftrightarrow x=\dfrac{2}{7}\\ b,\Leftrightarrow12x^2-4x^3+3x^3-12x^2=8\\ \Leftrightarrow-x^3=8=-2^3\\ \Leftrightarrow x=2\\ c,\Leftrightarrow4x^2\left(4x-2\right)-x^3+8x^2=15\\ \Leftrightarrow16x^3-8x^2-x^3+8x^2=15\\ \Leftrightarrow15x^3=15\\ \Leftrightarrow x^3=1\Leftrightarrow x=1\)

\(P=x\left(2x+1\right)-x^2\left(x+2\right)+x^3-x+3\\ P=2x^2+x-x^3-2x^2+x^3-x+3\\ P=3\left(đfcm\right)\)

a) \(P=3\left(x^2+2xy+y^2\right)-2\left(x+y\right)-100\)

\(P=3\left(x+y\right)^2-2.5-100\)

\(P=3.5^2-110\)

\(P=-35\)

b) \(Q=\left[x^3+y^3+3xy\left(x+y\right)\right]-2\left(x^2+2xy+y^2\right)+3.5+10\)

\(Q=\left(x+y\right)^3-2\left(x+y\right)^2+25\)

\(Q=5^3-2.5^2+25\)

\(Q=100\)

a, Ta có: \(\dfrac{a}{b}=\dfrac{c}{d}=>\dfrac{a}{c}=\dfrac{b}{d}=\dfrac{3a}{3c}\)

Áp dụng tính chất của day tỉ số bằng nhau ta được:

\(\dfrac{a}{c}=\dfrac{b}{d}=\dfrac{3a}{3c}=\dfrac{3a+b}{3c+d}\)

\(=>\dfrac{a}{c}=\dfrac{3a+b}{3c+d}=>\dfrac{a}{3a+b}=\dfrac{c}{3c+d}=>\left(đpcm\right)\)

Bài 1:

Ta có:\(\dfrac{a}{b}=\dfrac{c}{d}\Rightarrow\dfrac{a}{c}=\dfrac{b}{d}=\dfrac{3a}{3c}\)

Áp dụng tính chất của dãy tỉ số bằng nhau ta được:

\(\dfrac{a}{c}=\dfrac{b}{d}=\dfrac{3a}{3c}=\dfrac{3a+b}{3c+d}\)

(ĐPCM)

b, Ta có \(\dfrac{a}{b}=\dfrac{c}{d}=>\dfrac{a}{c}=\dfrac{b}{d}\)

Đặt \(\dfrac{a}{c}=\dfrac{b}{d}=x\)

Xét \(x^2=\dfrac{a^2}{c^2}=\dfrac{b^2}{d^2}=\dfrac{a^2-b^2}{c^2-d^2}=\dfrac{ab}{cd}\)

=>(đpcm)

a) cho A(x) = 0

\(=>2x^2-4x=0\)

\(x\left(2-4x\right)=0\Rightarrow\left[{}\begin{matrix}x=0\\4x=2\end{matrix}\right.\Rightarrow\left[{}\begin{matrix}x=0\\x=\dfrac{1}{2}\end{matrix}\right.\)

b)\(B\left(y\right)=4y-8\)

cho B(y) = 0

\(4y-8=0\Rightarrow4y=8\Rightarrow y=2\)

c)\(C\left(t\right)=3t^2-6\)

cho C(t) = 0

\(=>3t^2-6=0=>3t^2=6=>t^2=2\left[{}\begin{matrix}t=\sqrt{2}\\t=-\sqrt{2}\end{matrix}\right.\)

d)\(M\left(x\right)=2x^2+1\)

cho M(x) = 0

\(2x^2+1=0\Rightarrow2x^2=-1\Rightarrow x^2=-\dfrac{1}{2}\left(vl\right)\)

vậy M(x) vô nghiệm

e) cho N(x) = 0

\(2x^2-8=0\)

\(2\left(x^2-4\right)=0\)

\(2\left(x^2+2x-2x-4\right)=0\)

\(2\left(x-2\right)\left(x+2\right)=0\)

\(\Rightarrow\left[{}\begin{matrix}x-2=0\\x+2=0\end{matrix}\right.\Rightarrow\left[{}\begin{matrix}x=2\\x=-2\end{matrix}\right.\)

`e, N(x) = 2x^2 - 8 = 2( x^2 - 4 ) = 2( x-2 )( x + 2 )`

Xét `N(x)=0`

`=> 2(x-2)(x+2)=0`

`=>(x-2)(x+2)=0`

`=>x-2=0` hoặc `x+2=0`

`=>x=2` hoặc `x=-2`

Vậy `x in { +-2 }` là nghiệm của `N(x)` 

Vì x, y, z tỉ lệ với các số a, b, c nên  suy ra x = ka, y = kb, z = kc

Thay x = ka, y = kb, z = kc vào ( x 2   +   2 y 2   +   3 z 2 ) ( a 2   +   2 b 2   +   3 c 2 ) ta được

[ ( k a ) 2   +   2 ( k b ) 2   +   3 ( k c ) 2 ] ( a 2   +   2 b 2   +   3 c 2 )     =   ( k 2 a 2   +   2 k 2 b 2   +   3 k 2 c 2 ) ( a 2   +   2 b 2   +   3 c 2 )     =   k 2 ( a 2   +   2 b 2   +   3 c 2 ) ( a 2   +   2 b 2   +   3 c 2 )     =   k 2 ( a 2   +   2 b 2   +   3 c 2 ) 2     =   [ k ( a 2   +   2 b 2   +   3 c 2 ) ] 2       =   ( k a 2   +   2 k b 2   +   3 k c 2 ) 2       =   ( k a . a   +   2 k b . b   +   3 k c . c ) 2 =   ( x a   +   2 y b   +   3 z c ) 2  

do x = ka,y = kb, z = kc

Vậy

( x 2   +   2 y 2   +   3 z 2 ) ( a 2   +   2 b 2   +   3 c 2 )   =   ( a x   +   2 b y   +   3 c z ) 2

Đáp án cần chọn là: D