Hello everyone, I hope you can help me out. I'm typing this in a rush so please forgive me for not using LaTeX. Here is the problem:

Given the function f(x)=-0.00128x^3+0.144x^2-4.4x+60, find the point on f(x) whose coordinates have the greatest possible product.

If it's gonna be any help, here are the coefficients in fraction form (in the same order): -4/3125, 18/125, -22/5, and 60.

The graph does not pass through QIV, so I'm assuming the required point is in QI because points in QII and QIII have coordinates with opposite signs and points on the axes have at least one zero coordinate. That's easy enough, but I'm stumped beyond that point. Help in any form would be greatly appreciated. Thank you very much!

PS This is not a school assignment. I'm working as a math editor for a textbook publishing company. This problem came up, and I have to crack it to make sure that the problem is solvable. Too bad I don't see how it can be done. I really hope you can help me, but more than that, I hope this post doesn't get junked for violating Brilliant guidelines. Thanks again!

## Comments

Sort by:

TopNewestWhat have you tried? Where did you get stuck?

Log in to reply

– Francis Gerard Magtibay · 1 year, 10 months ago

First I tried multiplying f(x) by x to obtain another function that yields the product of the coordinates (let's call it g(x)), then I differentiated that function. So now I have g'(x). That's all I've done so far, but I'm not sure if it's correct. I'd continue by using derivatives to find maxima and minima, but then the problem would be off-topic and we'll have to transfer it to a more advanced section. Anyway, this is an interesting refresher because I haven't gone beyond algebra in a while. Thanks Sir Calvin!Log in to reply

In what section did they place this under? Is there the possibility of a typo? – Calvin Lin Staff · 1 year, 10 months ago

Log in to reply

Here is an overview of the given information. (I skipped the backstory, but the questions are copied verbatim.)

The following ordered pairs represent the number of kg of mangoes sold as a function of the price per kg: (50, 40), (0, 60), (20, 20), (75, 0).

EDIT: I'm done solving. Thanks for the hint, I'm getting really rusty. :) – Francis Gerard Magtibay · 1 year, 10 months ago

Log in to reply