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\(\frac{a-3}{a+3}=\frac{b-6}{b+6}\Rightarrow\left(a-3\right).\left(b+6\right)=\left(b-6\right).\left(a+3\right)\)
\(\Rightarrow ab+6a-3b-18=ab+3b-6a=18\)
\(\Rightarrow b.\left(a-3\right)+6.a-18=a.\left(b-6\right)+3.b-18\)
\(\Rightarrow b.\left(a-3\right)+6a=a.\left(b-6\right)+3b\)
\(\Rightarrow ab-3b=ab-6a+3b-6a\)
\(\Rightarrow ab-3b=ab-3.\left(4a-b\right)\)
\(b=4a-b\Rightarrow2b=4a\Rightarrow b=2a\Rightarrow\frac{a}{b}=\frac{1}{2}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
![](https://rs.olm.vn/images/avt/0.png?1311)
a) Ta có:
\(\begin{array}{l}\frac{6}{{10}} = \frac{{6:2}}{{10:2}} = \frac{3}{5};\\\frac{9}{{15}} = \frac{{9:3}}{{15:3}} = \frac{3}{5}\end{array}\)
\(\begin{array}{l}\frac{{6 + 9}}{{10 + 15}} = \frac{{15}}{{25}} = \frac{{15:5}}{{25:5}} = \frac{3}{5};\\\frac{{6 - 9}}{{10 - 15}} = \frac{{ - 3}}{{ - 5}} = \frac{3}{5}\end{array}\)
Ta được: \(\frac{{6 + 9}}{{10 + 15}} = \frac{{6 - 9}}{{10 - 15}} = \frac{6}{{10}} = \frac{9}{{15}}\)
b) - Vì \(k = \frac{a}{b} \Rightarrow a = k.b\)
Vì \(k = \frac{c}{d} \Rightarrow c = k.d\)
- Ta có:
\(\begin{array}{l}\frac{{a + c}}{{b + d}} = \frac{{k.b + k.d}}{{b + d}} = \frac{{k.(b + d)}}{{b + d}} = k;\\\frac{{a - c}}{{b - d}} = \frac{{k.b - k.d}}{{b - d}} = \frac{{k.(b - d)}}{{b - d}} = k\end{array}\)
- Như vậy, \(\frac{{a + c}}{{b + d}}\) =\(\frac{{a - c}}{{b - d}}\) = \(\frac{a}{b}\) =\(\frac{c}{d}\)( = k)
a: \(\dfrac{6+9}{10+15}=\dfrac{15}{25}=\dfrac{3}{5};\dfrac{6-9}{10-15}=\dfrac{-3}{-5}=\dfrac{3}{5}\)
=>Bằng nhau
b: a/b=c/d=k
=>a=bk; c=dk
\(\dfrac{a+c}{b+d}=\dfrac{bk+dk}{b+d}=k;\dfrac{a-c}{b-d}=\dfrac{bk-dk}{b-d}=k\)
=>\(\dfrac{a+c}{b+d}=\dfrac{a-c}{b-d}=\dfrac{a}{b}=\dfrac{c}{d}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
Vì các số a,b,c tỉ lệ nghịch với \(\frac{1}{2};\frac{1}{3};\frac{1}{4}\)nên
\(a:2=b:3=c:4\Rightarrow\frac{a}{2}=\frac{b}{3}=\frac{c}{4}\)
Đặt \(\frac{a}{2}=\frac{b}{3}=\frac{c}{4}=k\)nên \(a=2k;b=3k;c=4k\)
Khi đó \(M=\frac{\left(2a+3b+4c\right)^2}{a^2+b^2+c^2}=\frac{\left(2.2k+3.3k+4.4k\right)^2}{\left(2k\right)^2+\left(3k\right)^2+\left(4k\right)^2}\)
\(M=\frac{\left(4k+9k+16k\right)^2}{4k^2+9k^2+16k^2}\)
\(M=\frac{\left[k.\left(4+9+16\right)\right]^2}{k^2.\left(4+9+16\right)}\)
\(M=\frac{k^2.29^2}{k^2.29}=29\)
Vậy \(M=29\)