If 3f(x)+f(3-x)=x squared, what's f(x)?

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Homework Help Overview

The discussion revolves around finding a function f(x) that satisfies the equation 3f(x) + f(3-x) = x squared. Participants are exploring various substitutions and manipulations of the equation to derive f(x).

Discussion Character

  • Exploratory, Problem interpretation, Assumption checking

Approaches and Questions Raised

  • Participants attempt substitutions such as replacing x with y and x with 3-x to manipulate the original equation. There are discussions about isolating f(x) and f(3-x) through these substitutions. Some participants express confusion about the implications of certain substitutions and seek clarification on the reasoning behind them.

Discussion Status

The discussion is ongoing, with participants sharing their attempts and questioning the validity of their approaches. Some guidance has been offered regarding how to isolate f(x) using the derived equations, but there is no consensus on the next steps or the final form of f(x).

Contextual Notes

Participants are grappling with the implications of their substitutions and the resulting equations, indicating a need for clarity on the relationships between the variables involved. There is also mention of potential confusion arising from the nature of the substitutions being discussed.

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Homework Statement


Homework Equations


The Attempt at a Solution



I first tried replacing "x" when in parenthese with y. Therefor, 3y+3-y=x, yadda yadda, 2y=x-3, y=(x-3)/2, however, the real answer turned out to be some weird fractional thing. Please give me some kind of direction..

EDIT: Ignore that...
 
Last edited:
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pugfug90 said:
I first tried replacing "x" when in parenthese with y. Therefor, 3y+3-y=x, yadda yadda, 2y=x-3, y=(x-3)/2, however, the real answer turned out to be some weird fractional thing. Please give me some kind of direction..
OTHER EDIT!
Sorry! yes, my initial trick does work if you replace x by 3-x. I am obviously not thinking straight after having been teaching for 4 hours straight! just replace x by 3-x and then isolate your f(x)!


Ignore the rest...



EDIT: oops. I had read too quickly. My trick won't work right away. You will also have to consider replacing x by minus x to make it work

Start from the initial equation. Substitute x -> x-3 everywhere you see an x (on both sides of the equation. This will give you a new equation containing f(x) and f(x-3). Now use those two equations to get rid of f(3-x) to leave you with an answer for f(x)
 
Last edited:
3f(3-x)+f(x)=3-x is what I get if I plug in 3-x where x is...
 
pugfug90 said:
3f(3-x)+f(x)=3-x is what I get if I plug in 3-x where x is...

you mean (3-x)^2 on the right side??
 
Oops haha
So 3f(3-x)+f(x)=(x squared)+9-6x.

So can you tell me where to go from here? I tried substituting y into x again, get (x squared - 6x)/-2, while the real answer is something like x/3+x/5-3 or something. Also, where did the 'plug 3-x for x everywhere' thing came from? And if this is a "topic" I can look up like for quadratic equations, or "f and g compositions"?
 
pugfug90 said:
Oops haha
So 3f(3-x)+f(x)=(x squared)+9-6x.

So can you tell me where to go from here? I tried substituting y into x again, get (x squared - 6x)/-2, while the real answer is something like x/3+x/5-3 or something. Also, where did the 'plug 3-x for x everywhere' thing came from? And if this is a "topic" I can look up like for quadratic equations, or "f and g compositions"?

if you substitute again you will get back to the initial equation. No, just use the two equations you now have. get rid of f(3-x) (isolate it from one equation and plug in the other one) and you can isolate f(x) now.
 
I am sorry, I was just reading through this thread: how can you substitute x for x-3? Doesn't this imply -3=0?
 
qspeechc said:
I am sorry, I was just reading through this thread: how can you substitute x for x-3? Doesn't this imply -3=0?
He said substitute for, not set equal to.

If it makes you feel better, try the (equivalent) substitution x -> y - 3.
 
3f(3-x)+f(x)=(x squared)+9-6x

get rid of f(3-x) (isolate it from one equation and plug in the other one) and you can isolate f(x) now.
===
If I want to isolate f(x).. it'd be f(x)=xsquared+9-6x-3f(3-x), right?
 

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