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Olympiad workbook
Exam
Olympiad workbook
Subject
IOQM
Chapter
Basics of Mathematics

Olympiad workbook · IOQM · Basics of Mathematics

The positive real numbers a, b, c satisfy: aligned & (a)/(2 b+1)+(2...

The positive real numbers a,b,ca, b, c satisfy: a2b+1+2b3c+1+3ca+1=11a+1+12b+1+13c+1=2\begin{aligned} & \frac{a}{2 b+1}+\frac{2 b}{3 c+1}+\frac{3 c}{a+1}=1 \\ & \frac{1}{a+1}+\frac{1}{2 b+1}+\frac{1}{3 c+1}=2 \end{aligned} What is the value of 1a+1b+1c?\frac{1}{a}+\frac{1}{b}+\frac{1}{c} ?

Correct answer

12

Step-by-step solution

3ca+1+a2b+1+2b3c+1=1\frac{3 c}{a+1}+\frac{a}{2 b+1}+\frac{2 b}{3 c+1}=1......(i) 1a+1+12b+1+13c+1=2\frac{1}{a+1}+\frac{1}{2 b+1}+\frac{1}{3 c+1}=2......(ii) Add (i) and (ii) ⇒3c+1a+1+a+12b+1+2b+13c+1=3\Rightarrow \quad \frac{3 c+1}{a+1}+\frac{a+1}{2 b+1}+\frac{2 b+1}{3 c+1}=3 ⇒ A.M. ≥ G.M. ⇒(3c+1a+1+a+12b+1+2b+13c+13)≥(1)13⇒3c+1a+1=1 and 2b+13c+1=1a+12b+1=1⇒(a+1)=(2b+1)=(3(+1))=k⇒1k+1k+1k=2⇒3k=2⇒k=32⇒a=12,b=14,c=16⇒1a+1b+1c=2+4+6=12\begin{aligned} & \Rightarrow \quad \text { A.M. } \geq \text { G.M. } \\ & \Rightarrow \quad\left(\frac{\frac{3 c+1}{a+1}+\frac{a+1}{2 b+1}+\frac{2 b+1}{3 c+1}}{3}\right) \geq(1)^{\frac{1}{3}} \\ & \Rightarrow \quad \frac{3 c+1}{a+1}=1 \text { and } \frac{2 b+1}{3 c+1}=1 \\ & \quad \frac{a+1}{2 b+1}=1 \\ & \Rightarrow \quad(a+1)=(2 b+1)=(3(+1))=k \\ & \Rightarrow \quad \frac{1}{k}+\frac{1}{k}+\frac{1}{k}=2 \Rightarrow \frac{3}{k}=2 \Rightarrow k=\frac{3}{2} \\ & \Rightarrow \quad a=\frac{1}{2}, b=\frac{1}{4}, c=\frac{1}{6} \\ & \Rightarrow \quad \frac{1}{a}+\frac{1}{b}+\frac{1}{c}=2+4+6=12\end{aligned}
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