Maximum Value of Eh?

Algebra Level 4

a 2 + b 2 + c 2 = 2014 a + 2 b + 3 c = 86 \begin{aligned} a^{2}+b^{2}+c^{2}&=&2014 \\ a+2b+3c&=&86 \end{aligned}

Let a , b , c a,b,c be real numbers such that they satisfy the system of equations above.

The maximum value of a a can be expressed as m n \dfrac{m}{n} , where m m and n n are coprime positive integers. Find the value of m + n m+n .


The answer is 310.

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

The first thing we can do is to express b b and c c in terms of a a .

b 2 + c 2 b^{2} + c^{2} = 2014 a 2 2014 - a^{2} ( 1 ) (1)

2 b + 3 c 2b + 3c = 86 a 86 - a ( 2 ) (2)

Then applying the Cauchy- Schwarz Inequality (As in the tag), We will have:

( 2 2 + 3 2 ) 2^{2} + 3^{2}) ( b 2 + c 2 ) b^{2} + c^{2}) \geq ( 2 b + 3 c ) 2 (2b+3c)^{2}

Substituting ( 1 ) (1) and ( 2 ) (2) ,

( 13 ) (13) ( 2014 a 2 ) (2014 - a^{2}) \geq ( 86 a ) 2 (86 - a)^{2}

Expanding and Manipulating, we will get:

0 7 a 2 86 a 9393 0 \geq 7 a^{2} - 86 a - 9393

Which gives the range of possible values for a which is:

31 a 303 7 -31 \leq a \leq \frac{303}{7}

The maximum value of a is 303 7 \frac{303}{7} and gives m = 303 , n = 7 m = 303, n = 7 and the required answer which is m + n = 310 \boxed{m+n = 310}

Yes! The exact solution I was looking for! :)

Sean Ty - 6 years, 10 months ago

Log in to reply

Can't you give some smaller value in question @Sean Ty

Prakash Chandra Rai - 6 years, 2 months ago

Yes, I did the same here. I wonder if there's another way of solving this. Is it possible to solve by using Lagrange's multipliers? I've tried but I did not succeed.

Dieuler Oliveira - 6 years, 10 months ago

Yes, cauchy schwarz will pull out the trick.

Jun Arro Estrella - 4 years, 4 months ago

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...