Are you smarter than me? 45

Algebra Level 3

If a , b , c \color{#3D99F6}{a,b,c} are positive integers

Find minimum value of

a 2 + 1 b + c + b 2 + 1 a + c + c 2 + 1 b + a \color{darkorange}{\dfrac{a^2+1}{b+c}+\dfrac{b^2+1}{a+c}+\dfrac{c^2+1}{b+a}}


The answer is 3.

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

1 solution

Hem Shailabh Sahu
Feb 15, 2015

a 2 + 1 b + c + b 2 + 1 a + c + c 2 + 1 a + b \frac{a^2+1}{b+c}+\frac{b^2+1}{a+c}+\frac{c^2+1}{a+b}

= a ( a + 1 a ) b + c + b ( b + 1 b ) a + c + c ( c + 1 c ) a + b =\frac{a(a+\frac{1}{a})}{b+c}+\frac{b(b+\frac{1}{b})}{a+c}+\frac{c(c+\frac{1}{c})}{a+b}

As a , b , c a,b,c are positive integers, from the Arithmetic Mean & Geometric Mean Relation for a positive real numbers x x & 1 x \frac{1}{x} :

x + 1 x 2 x+\frac{1}{x}\geq2

As it is observed, the minimum value that x + 1 x x+\frac{1}{x} can attain is 2 2 , for x = 1 x=1 . This is applicable for x = a , b , c x=a,b,c .

Therefore, for the minimum value of the expression, a = b = c = 1 \boxed{a=b=c=1} .

Minimum value of the given expression is

min ( a 2 + 1 b + c + b 2 + 1 a + c + c 2 + 1 a + b ) = 3 ( 1 + 1 1 + 1 ) = 3 \Rightarrow \min(\frac{a^2+1}{b+c}+\frac{b^2+1}{a+c}+\frac{c^2+1}{a+b})=3*(\frac{1+1}{1+1})=\boxed{3}

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...