# Setting Derivative = 0 and solving

• deedsy
In summary, the conversation discusses the legality of multiplying a fraction by zero when setting the derivative equal to zero and solving for a variable. It is determined that this is acceptable, and an alternative method of dividing the left side by x/x is also mentioned.

## Homework Statement

I'm currently working on a problem that requires me to set the derivative = 0 and solve for a variable (call it x). The derivative comes out to be a fraction, with x terms in both the numerator and denominator. Is it legal to just multiply 0 by the denominator (thereby canceling it) even if it has the term of interest as part of it?

Simple Ex: say the derivative came out to be x-3 / 2x. And I want to solve for x.
When I set that derivative equal to zero, can i just multiply 0 by 2x, leaving x-3=0? So x=3

none

## The Attempt at a Solution

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That should be ok. You could also divide the left side by x / x, assuming that x is not equal to zero, resulting in:

( 1 - (3/x) ) / 2 = 0

• 1 person
thank you

rcgldr said:
That should be ok. You could also divide the left side by x / x, assuming that x is not equal to zero, resulting in:

( 1 - (3/x) ) / 2 = 0

i thought they're supposed to show the work?

MGCLO said:
i thought they're supposed to show the work?

It is a requirement for HW problems, but my question was geared towards a concept. The equation I'm deriving for the HW would probably take up an entire line on here. The example I put on was just that, an example, it wasn't even close to my actual problem (although I wish it was haha)

MGCLO said:
i thought they're supposed to show the work?
Multiplying both sides by 2x is just as valid as dividing the left side by x/x. I only showed that as an alternative in case there's a situation where that would be a better option for a different equation.