A problem from the quiz

[Soumyadeep mandal]

[Jatin Kr Dey]

Try to use $$ \displaystyle (a^2+b^2+c^2-ab-bc-ca)= \frac{1}{2} [(a-b)^2+(b-c)^2+(c-a)^2] $$

 

then proceed by the RMS - AM   ineqality .

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