min. of log expression

\hspace{-16}$If $\mathbf{a\;,b\;c\geq 2\;,}$ Then find Min. value of \\\\ $\mathbf{\log_{b+c}a+\log_{c+a}b+\log_{a+b}c}$

5 Answers

21
Shubhodip ·

Hi,I have a solution, I think it is correct...Check it and do tell me...

c\geq 2 \implies c-1 \geq \frac c 2 \geq \frac c b

\implies bc\geq b+ c, \implies \ln b + \ln c \geq \ln(b+c)

\implies \frac{1}{\ln(b+c)}\geq \frac{1}{\ln b + \ln c}

\implies \log _{b+c}a= \frac{\ln a}{\ln(b+c)}\geq \frac{\ln a}{\ln b + \ln c}

\implies \sum_{cyc}^{{}}\log _{b+c}a= \sum_{cyc}^{{}}\frac{\ln a}{\ln(b+c)}\geq \sum_{cyc}^{{}}\frac{\ln a}{\ln b + \ln c}\geq \frac{3}{2}

The last step being true by Nesbitt's Inequality...

Equality holds when a=b=c=2

71
Vivek @ Born this Way ·

Subhodip, Can't we assert the third step using that ' ln x ' in concave function.

21
Shubhodip ·

lnx is concave which gives ln(b+c/2)≥ (lna + lnb)/2

so i dont think we can....??

71
Vivek @ Born this Way ·

hmm..

1708
man111 singh ·

thanks Shubodip

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