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Do \(a,b,c\ne0\)
\(\dfrac{ab}{a+b}=\dfrac{bc}{b+c}=\dfrac{ac}{a+c}\Rightarrow\dfrac{a+b}{ab}=\dfrac{b+c}{bc}=\dfrac{a+c}{ac}\)
\(\Rightarrow\dfrac{a}{ab}+\dfrac{b}{ab}=\dfrac{b}{bc}+\dfrac{c}{bc}=\dfrac{a}{ac}+\dfrac{c}{ac}\)
\(\Rightarrow\dfrac{1}{a}+\dfrac{1}{b}=\dfrac{1}{b}+\dfrac{1}{c}=\dfrac{1}{a}+\dfrac{1}{c}\) \(\Rightarrow\left\{{}\begin{matrix}\dfrac{1}{a}+\dfrac{1}{b}=\dfrac{1}{b}+\dfrac{1}{c}\\\dfrac{1}{b}+\dfrac{1}{c}=\dfrac{1}{a}+\dfrac{1}{c}\end{matrix}\right.\)
\(\Rightarrow\left\{{}\begin{matrix}\dfrac{1}{a}=\dfrac{1}{c}\\\dfrac{1}{b}=\dfrac{1}{a}\end{matrix}\right.\) \(\Rightarrow\left\{{}\begin{matrix}a=c\\b=a\end{matrix}\right.\) \(\Rightarrow a=b=c\)
\(\Rightarrow M=\dfrac{a.a+a.a+a.a}{a^2+a^2+a^2}=\dfrac{3a^2}{3a^2}=1\)
\(\dfrac{ab}{a+b}=\dfrac{bc}{b+c}=\dfrac{ac}{a+c}\Rightarrow\dfrac{a+b}{ab}=\dfrac{b+c}{bc}=\dfrac{a+c}{ac}\)
\(\Rightarrow\dfrac{1}{a}+\dfrac{1}{b}=\dfrac{1}{b}+\dfrac{1}{c}=\dfrac{1}{a}+\dfrac{1}{c}\)
\(\Rightarrow\left\{{}\begin{matrix}\dfrac{1}{a}+\dfrac{1}{b}=\dfrac{1}{b}+\dfrac{1}{c}\\\dfrac{1}{b}+\dfrac{1}{c}=\dfrac{1}{a}+\dfrac{1}{c}\end{matrix}\right.\) \(\Rightarrow\left\{{}\begin{matrix}\dfrac{1}{a}=\dfrac{1}{c}\\\dfrac{1}{b}=\dfrac{1}{a}\end{matrix}\right.\) \(\Rightarrow\left\{{}\begin{matrix}a=c\\a=b\end{matrix}\right.\) \(\Rightarrow a=b=c\)
Thay vào M ta được:
\(M=\dfrac{ab+bc+ac}{a^2+b^2+c^2}=\dfrac{a.a+a.a+a.a}{a^2+a^2+a^2}=\dfrac{3a^2}{3a^2}=1\)
theo đề bài ta có:
\(\Rightarrow\dfrac{abc}{ab+bc}=\dfrac{abc}{ab+ac}=\dfrac{abc}{bc+ab}\)
\(\Rightarrow ac+bc=ab+ac=bc+ab\)
\(\Rightarrow M=\dfrac{ab+bc+ca}{a^2+b^2+c^2}=\dfrac{a^2+b^2+c^2}{a^2+b^2+c^2}=1\)
Ta có \(\dfrac{ab}{a+b}\)=\(\dfrac{bc}{b+c}\)=\(\dfrac{ca}{c+a}\)
\(=>\)\(\dfrac{a+b}{ab}\)=\(\dfrac{b+c}{bc}\)=\(\dfrac{c+a}{ca}\)
\(=>\)\(\dfrac{1}{a}\)+\(\dfrac{1}{b}\)=\(\dfrac{1}{b}\)+\(\dfrac{1}{c}\)=\(\dfrac{1}{c}\)+\(\dfrac{1}{a}\)
\(=>\)\(\dfrac{1}{b}\)+\(\dfrac{1}{a}\)=\(\dfrac{1}{c}\)+\(\dfrac{1}{b}\)
\(\dfrac{1}{c}\)+\(\dfrac{1}{b}\)=\(\dfrac{1}{a}\)+\(\dfrac{1}{c}\)
\(\dfrac{1}{a}\)+\(\dfrac{1}{c}\)=\(\dfrac{1}{b}\)+\(\dfrac{1}{a}\)
\(=>\)\(\dfrac{1}{a}=\dfrac{1}{b}=\dfrac{1}{c}\)
\(=>\)a=b=c
Vậy: M=\(\dfrac{ab+bc+ca}{a^2+b^2+c^2}=\dfrac{a^2+a^2+a^2}{a^2+a^2+a^2}\)
= 1
\(\dfrac{ab}{a+b}=\dfrac{bc}{b+c}=\dfrac{ca}{c+a}\)
=> \(\dfrac{abc}{ac+bc}=\dfrac{abc}{ab+ac}=\dfrac{abc}{bc+ab}\)
=> ac + bc = ab + ac = bc + ab (do abc \(\ne0\))
=> ac + bc - ab - ac = 0
=> bc - ab = 0
=> b(c - a) = 0
Mà b \(\ne0\) nên c - a = 0 => c = a
Tương tự ta có: a = b
Từ đó có: a = b = c
Thay vào M được:
\(M=\dfrac{a^2+a^2+a^2}{a^2+a^2+a^2}=1\)
\(\dfrac{ab}{a+b}=\dfrac{bc}{b+c}=\dfrac{ca}{c+a}\Rightarrow\dfrac{a+b}{ab}=\dfrac{b+c}{bc}=\dfrac{c+a}{ca}\)
\(\Rightarrow\dfrac{1}{a}+\dfrac{1}{b}=\dfrac{1}{b}+\dfrac{1}{c}=\dfrac{1}{c}+\dfrac{1}{a}\)
\(\Rightarrow\left\{{}\begin{matrix}\dfrac{1}{a}=\dfrac{1}{c}\\\dfrac{1}{a}=\dfrac{1}{b}\end{matrix}\right.\) \(\Rightarrow a=b=c\)
\(\Rightarrow M=\dfrac{a^2+a^2+a^2}{a^2+a^2+a^2}=1\)
\(B=\dfrac{1}{1+a+ab}+\dfrac{1}{1+b+bc}+\dfrac{1}{1+c+ca}=\dfrac{1}{1+a+ab}+\dfrac{a}{a+ab+abc}+\dfrac{ab}{ab+abc+abca}\)
vì abc =1 nên B=\(\dfrac{1}{1+a+ab}+\dfrac{a}{a+ab+1}+\dfrac{ab}{ab+1+a}=\dfrac{1+a+ab}{a+1+ab}=1\)
chúc bạn học tót ^^
\(\frac{ab}{a+b}=\frac{bc}{b+c}=\frac{a}{a+b}\cdot b=\frac{c}{b+c}\cdot b\)
\(\Rightarrow\frac{a}{a+b}=\frac{c}{b+c}\Rightarrow a\left(b+c\right)=c\left(a+b\right)\Rightarrow ab+ac=ac+bc\Rightarrow ab=bc\Rightarrow a=c\left(1\right)\)
\(\frac{ab}{a+b}=\frac{ac}{a+c}=\frac{b}{a+b}\cdot a=\frac{c}{a+c}\cdot a\)
\(\Rightarrow\frac{b}{a+b}=\frac{c}{a+c}\Rightarrow b\left(a+c\right)=c\left(a+b\right)\Rightarrow ab+bc=ac+bc\Rightarrow ab=ac\Rightarrow b=c\left(2\right)\)
\(\frac{bc}{b+c}=\frac{ac}{a+c}=\frac{b}{b+c}\cdot c=\frac{a}{a+c}\cdot c\)
\(\Rightarrow\frac{b}{b+c}=\frac{a}{a+c}\Rightarrow b\left(a+c\right)=a\left(b+c\right)\Rightarrow ab+bc=ab+ac\Rightarrow bc=ac\Rightarrow a=b\left(3\right)\)
từ \(\left(1\right)\left(2\right)\left(3\right)\Rightarrow a=b=c\)
\(\Rightarrow M=\frac{ab+bc+ac}{a^2+b^2+c^2}=\frac{a^2+b^2+c^2}{a^2+b^2+c^2}=1\)
Ta có:\(\frac{ab}{a+b}=\frac{bc}{b+c}=\frac{ca}{c+a}\)
\(\iff\)\(\frac{abc}{ac+bc}=\frac{abc}{ab+ac}=\frac{abc}{bc+ba}\)
\(\iff\) \(ac+bc=ab+ac=bc+ba\)
+)\(ac+bc=ab+ac\)
\(\implies\)\(bc=ab\)
\(\implies\) \(c=a\left(1\right)\)
+)\(ab+ac=bc+ba\)
\(\implies\) \(ac=bc\)
\(\implies\) \(a=b\left(2\right)\)
Từ \(\left(1\right);\left(2\right)\)
\(\implies\) \(a=b=c\)
\(\implies\) \(M=\frac{ab+bc+ca}{a^2+b^2+c^2}=\frac{aa+bb+cc}{a^2+b^2+c^2}=\frac{a^2+b^2+c^2}{a^2+b^2+c^2}=1\)
Vậy \(M=1\)
Lời giải:
\(\frac{ab}{a+b}=\frac{bc}{b+c}=\frac{ca}{c+a}\Rightarrow \frac{abc}{c(a+b)}=\frac{abc}{a(b+c)}=\frac{bca}{b(c+a)}\)
\(\Leftrightarrow c(a+b)=a(b+c)=b(c+a)\)
\(\Leftrightarrow ac+bc=ab+ac=bc+ab\Leftrightarrow ab=bc=ac\)
\(\Rightarrow a=b=c\) (do $a,b,c>0$)
$\Rightarrow M=\frac{a^2+a^2+a^2}{a^2+a^2+a^2}=1$