Minimium

Algebra Level 3

If a a , b b , and c c are positive real numbers such that a 2 + b 2 + c 2 = 1 a^2+b^2+c^2=1 , then find the minimum value of the following expression a 2 1 + 2 b c + b 2 1 + 2 c a + c 2 1 + 2 a b \frac{a^2}{1+2bc}+\frac{b^2}{1+2ca}+\frac{c^2}{1+2ab}


The answer is 0.6.

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1 solution

Marco Brezzi
Aug 6, 2017

Relevant wiki: Titu's Lemma

Since a , b , c > 0 a,b,c>0 we can apply Titu's Lemma

a 2 1 + 2 b c + b 2 1 + 2 a c + c 2 1 + 2 a b ( a + b + c ) 2 3 + 2 a b + 2 a c + 2 a b \dfrac{a^2}{1+2bc}+\dfrac{b^2}{1+2ac}+\dfrac{c^2}{1+2ab}\geq \dfrac{(a+b+c)^2}{3+2ab+2ac+2ab}

Equality holds when a = b = c = 1 3 a=b=c=\dfrac{1}{\sqrt{3}} , substituting we get

a 2 1 + 2 b c + b 2 1 + 2 a c + c 2 1 + 2 a b ( 3 1 3 ) 2 3 + 3 2 ( 3 ) 2 3 5 = 0.6 \begin{aligned} \dfrac{a^2}{1+2bc}+\dfrac{b^2}{1+2ac}+\dfrac{c^2}{1+2ab} & \geq \frac{\left(3\cdot\dfrac{1}{\sqrt{3}}\right)^2}{3+3\cdot\dfrac{2}{(\sqrt{3})^2}}\\ & \geq\dfrac{3}{5}=\boxed{0.6} \end{aligned}

Note: \textbf{Note:}

I'm assuming there is a typo in the text of the problem, since if you don't give the constraint that they are positive reals there is no global minimum

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