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1, \(\dfrac{a}{b+c+d}=\dfrac{b}{a+c+d}=\dfrac{c}{a+b+d}=\dfrac{d}{a+b+c}=\dfrac{a+b+c+d}{3\left(a+b+c+d\right)}=\dfrac{1}{3}\)
Do đó \(\left\{{}\begin{matrix}3a=b+c+d\left(1\right)\\3b=a+c+d\left(2\right)\\3c=a+b+d\left(3\right)\\3d=a+b+c\left(4\right)\end{matrix}\right.\)
Từ (1) và (2) \(\Rightarrow3\left(a+b\right)=a+b+2c+2d\Leftrightarrow2\left(a+b\right)=2\left(c+d\right)\Leftrightarrow a+b=c+d\Leftrightarrow\dfrac{a+b}{c+d}=1\)
Tương tự cũng có: \(\dfrac{b+c}{a+d}=1;\dfrac{c+d}{a+b}=1;\dfrac{d+a}{b+c}=1\)
\(\Rightarrow A=4\)
2, Có \(\dfrac{x^3}{8}=\dfrac{y^3}{64}=\dfrac{z^3}{216}\Leftrightarrow\dfrac{x}{2}=\dfrac{y}{4}=\dfrac{z}{6}\)\(\Leftrightarrow\dfrac{x^2}{4}=\dfrac{y^2}{16}=\dfrac{z^2}{36}=\dfrac{x^2+y^2+z^2}{4+16+36}=\dfrac{14}{56}=\dfrac{1}{4}\)
Do đó \(\dfrac{x^2}{4}=\dfrac{1}{4};\dfrac{y^2}{16}=\dfrac{1}{4};\dfrac{z^2}{36}=\dfrac{1}{4}\)
\(\Rightarrow\left\{{}\begin{matrix}x^2=1\\y^2=4\\z^2=9\end{matrix}\right.\)\(\Rightarrow\left\{{}\begin{matrix}x=\pm1\\y=\pm2\\z=\pm3\end{matrix}\right.\)
Vậy \(\left(x;y;z\right)=\left(1;2;3\right),\left(-1;-2;-3\right)\)
Bài 2 :
a, Ta có : \(\dfrac{x^3}{8}=\dfrac{y^3}{64}=\dfrac{z^3}{216}\)
\(\Rightarrow\dfrac{x}{2}=\dfrac{y}{4}=\dfrac{z}{6}\)
\(\Rightarrow\dfrac{x^2}{4}=\dfrac{y^2}{16}=\dfrac{z^2}{36}=\dfrac{x^2+y^2+z^2}{4+16+36}=\dfrac{1}{4}\)
\(\Rightarrow\left\{{}\begin{matrix}x^2=1\\y^2=4\\z^2=9\end{matrix}\right.\)
\(\Rightarrow\left\{{}\begin{matrix}x=\pm1\\y=\pm2\\z=\pm3\end{matrix}\right.\)
Vậy ...
b, Ta có : \(\dfrac{2x+1}{5}=\dfrac{3y-2}{7}=\dfrac{2x+3y-1}{5+7}=\dfrac{2x+3y-1}{6x}\)
\(\Rightarrow6x=12\)
\(\Rightarrow x=2\)
\(\Rightarrow y=3\)
Vậy ...
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Giải:
Áp dụng tính chất dãy tỉ số bằng nhau ta có:
\(\frac{b+c+1}{a}=\frac{a+c+2}{b}=\frac{a+b-3}{c}=\frac{b+c+1+a+c+2+a+b-3}{a+b+c}=\frac{2a+2b+2c}{a+b+c}=\frac{2\left(a+b+c\right)}{a+b+c}=2=\frac{1}{a+b+c}\)
Có: \(2=\frac{1}{a+b+c}\Rightarrow a+b+c=\frac{1}{2}\)
Xét \(\frac{b+c+1}{a}=2\Rightarrow b+c+1=2a\)
\(\Rightarrow a+b+c+1=3a\)
\(\Rightarrow\frac{1}{2}+1=3a\)
\(\Rightarrow3a=\frac{3}{2}\)
\(\Rightarrow a=\frac{1}{2}\)
Xét \(\frac{a+c+2}{b}=2\Rightarrow a+c+2=2b\)
\(\Rightarrow a+b+c+2=3b\)
\(\Rightarrow\frac{1}{2}+2=3b\)
\(\Rightarrow\frac{5}{2}=3b\)
\(\Rightarrow b=\frac{5}{6}\)
Xét \(\frac{a+b-3}{c}=2\Rightarrow a+b-3=2c\)
\(\Rightarrow a+b+c-3=3c\)
\(\Rightarrow\frac{1}{2}-3=3c\)
\(\Rightarrow\frac{-5}{2}=3c\)
\(\Rightarrow c=\frac{-5}{6}\)
Vậy bộ số \(\left(a;b;c\right)\) là \(\left(\frac{1}{2};\frac{5}{6};\frac{-5}{6}\right)\)
\(\frac{b+c+1}{a}=\frac{a+c+2}{b}=\frac{a+b-3}{c}=\frac{b+c+1+a+c+2+a+b-3}{a+b+c}=2\)(T/C...)
\(\Rightarrow\frac{1}{a+b+c}=2\Rightarrow a+b+c=\frac{1}{2}=0,5\)
\(\Rightarrow\frac{b+c+1}{a}=2\Rightarrow\frac{0,5-a+1}{a}=2\Rightarrow1,5-a=2a\Rightarrow a=\frac{1}{2}\)
\(\Rightarrow\frac{a+c+2}{b}=2\Rightarrow\frac{0,5-b+2}{b}=2\Rightarrow2,5-b=2b\Rightarrow b=\frac{5}{6}\)
\(\Rightarrow c=0,5-\frac{1}{2}-\frac{5}{6}=-\frac{5}{6}\)
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áp dụng t.c dãy tỉ số bằng nhau ta có:
\(\frac{b+c+1}{a}=\frac{a+c+2}{b}=\frac{a+b-3}{c}=\frac{b+c+1+a+c+2+a+b-3}{a+b+c}=2\)(vì a+b+c khác 0)
\(\Rightarrow\frac{1}{a+b+c}=2\Rightarrow a+b+c=\frac{1}{2}\)
\(\frac{b+c+1}{a}=2\Rightarrow2a=b+c+1\Rightarrow3a=a+b+c+1\Rightarrow a=\frac{1}{2}\)
\(\frac{a+c+2}{b}=2\Rightarrow2b=a+c+2\Rightarrow3b=a+b+c+2\Rightarrow b=\frac{5}{6}\)
\(\frac{a+b-3}{c}=2\Rightarrow2c=a+b-3\Rightarrow3c=a+b+c-3\Rightarrow c=-\frac{5}{6}\)
Vậy \(a=\frac{1}{2},b=\frac{5}{6},c=-\frac{5}{6}\)
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Áp dụng tính chất dãy tỉ số bằng nhau ta có:
Nếu a + b + c = 0 => a = b = c = 0
Nếu a + b + c khác 0
Áp dụng dãy tỉ số bằng nhau
\(\frac{a}{b+c-5}=\frac{b}{a+c+3}=\frac{c}{a+b+2}=\frac{a+b+c}{2a+2b+2c}=\frac{1}{2}\)
=> \(\frac{1}{2}\left(a+b+c\right)=\frac{1}{2}\Rightarrow a+b+c=1\)
=> \(\hept{\begin{cases}b+c=1-a\\b+a=1-c\\a+c=1-b\end{cases}}\)
Khi đó ta có: \(\frac{a}{1-a-5}=\frac{b}{1-b+3}=\frac{c}{1-c+2}=\frac{1}{2}\)
=> \(\frac{a}{-a-4}=\frac{b}{-b+4}=\frac{c}{-c+3}=\frac{1}{2}\)
=> a = -4/3; b = 4/3; c = 1
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bài này dài dòng lắm tóm lại a=1/2,b=5/6,z=-5/6