S8.3.7.4. The sum of two positive numbers is 16.

Click For Summary

Discussion Overview

The discussion revolves around finding the smallest possible value of the sum of the squares of two positive numbers whose sum is 16. Participants explore different methods to approach the problem, including calculus and algebraic manipulation.

Discussion Character

  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant begins by expressing the relationship between the two numbers and sets up the equation for the sum of their squares.
  • Another participant encourages continuation of the solution process.
  • A participant derives the expression for the sum of squares and finds the critical point by taking the derivative, suggesting that the minimum occurs at x=8, yielding a sum of squares of 128.
  • There is a question raised about whether the value found is indeed the smallest for x, indicating a potential misunderstanding of the problem's requirements.
  • A later reply acknowledges the confusion and suggests that further proof is needed to confirm the minimum value.
  • Another participant proposes an alternative method of completing the square to find the minimum, arriving at the same conclusion of 128 when x=8.

Areas of Agreement / Disagreement

Participants express uncertainty about whether the value found is the smallest and whether the original question was interpreted correctly. There is no consensus on the necessity of proving the minimum value.

Contextual Notes

Participants have not resolved the need for a proof of the minimum value, and there are differing opinions on the methods used to arrive at the solution.

karush
Gold Member
MHB
Messages
3,240
Reaction score
5
3.7.4. The sum of two positive numbers is 16.

What is the smallest possible value of the sum of their squares?

$x+y=16\implies y=16-x$
Then
$x^2+(16-x)^2=2 x^2 - 32x + 256$

So far
... Hopefully
 
Last edited:
Physics news on Phys.org
keep going ... ATQ
 
[3.7.4. The sum of two positive numbers is 16.
What is the smallest possible value of the sum of their squares?

$x+y=16\implies y=16-x$
Then
$z=x^2+(16-x)^2=2 x^2 - 32x + 256$
Then
$z'=4x-32$
So
$z'(x)=0\quad x=8$
Check
8^2+8^2=128
 
karush said:
[3.7.4. The sum of two positive numbers is 16.
What is the smallest possible value of the sum of their squares?

$x+y=16\implies y=16-x$
Then
$z=x^2+(16-x)^2=2 x^2 - 32x + 256$
Then
$z'=4x-32$
So
$z'(x)=0\quad x=8$
Check
8^2+8^2=128
Question: Is that the smallest value for x?

-Dan
 
topsquark said:
Question: Is that the smallest value for x?

-Dan

Question: Is that what was asked in the question? I thought they were asking for the smallest sum of squares...
 
Prove It said:
Question: Is that what was asked in the question? I thought they were asking for the smallest sum of squares...
Okay. Yes, you are right. I was trying to get karush to prove that this was the minimum answer in order to round out his solution. He never proved that.

-Dan
 
Rather than use something as "advanced" as setting the derivative to 0, I would "complete the square". [math]2x^2- 32x+ 256= 2(x^2- 16x+ 128)= 2(x^2- 16x+ 64+ 64)= 2((x- 8)^2+ 64)[/math].

That will be smallest when x= 8 and then the value will be 2(64)= 128.
 
Last edited by a moderator:

Similar threads

Replies
4
Views
2K
  • · Replies 5 ·
Replies
5
Views
1K
  • · Replies 2 ·
Replies
2
Views
2K
  • · Replies 6 ·
Replies
6
Views
2K
  • · Replies 1 ·
Replies
1
Views
3K
  • · Replies 3 ·
Replies
3
Views
1K
  • · Replies 1 ·
Replies
1
Views
2K
  • · Replies 5 ·
Replies
5
Views
4K
  • · Replies 20 ·
Replies
20
Views
3K
  • · Replies 3 ·
Replies
3
Views
1K