# Thread: Simple Formula Help Needed

1. ## Simple Formula Help Needed

Again, a simple fix is all this will take but it is frustrating. Why a textbook doesn't clearly point out where you divide and where you substract instead of divide is beyond mysterious. Also, I'm working a little ahead of the class right now so no access to the professor.

So here goes:

Solving for x; Ax+By=c

What I did was divide By by itself to get c over By. This leaves Ax= c over By. Then I subtracted A from itself to get a final answer of x = c over By -A.

But that's incorrect. Apparently I should have subtracted By from itself to get c-By. Then divided A by itself for a final answer of c-By over A

So what's the rule about when you subtract and when you divide? I have chemistry coming up in the fall so I'm especially keen to get this right. And of course I'll reiterate my current disdain for math textbooks leaving out simple but very key information.

2. Oh, and while I'm at it, another mystery equation is next up. Again, it seems I'm simply missing a simple piece of information that would basically unlock the rest of the assignment.

Solve for c: p=ab over c

I don't know where to start on this one. I assume that when information isn't given in a textbook but a problem is presented that you haven't been given information on, it's supposed to make you think. But what if you're just not that smart?

3. Originally Posted by Ingersoll
Oh, and while I'm at it, another mystery equation is next up. Again, it seems I'm simply missing a simple piece of information that would basically unlock the rest of the assignment.

Solve for c: p=ab over c

I don't know where to start on this one. I assume that when information isn't given in a textbook but a problem is presented that you haven't been given information on, it's supposed to make you think. But what if you're just not that smart?
Solving an equation means that you isolate the variable in question on one side of the equal sign and all the other stuff on the other side.

You can cancel an operation by using it's inverse operation. That means:

If you want to cancel anaddition use subtraction;
if you want to cancel a multiplication use division
and vice versa.

$\displaystyle p=\dfrac{ab}{c}$
1. c should stand alone at the LHS, that means you have to cancel a division. So use multiplication on both sides of the equation!

$\displaystyle p \cdot c=\dfrac{ab}{c} \cdot c$

$\displaystyle p \cdot c=ab \cdot \dfrac{c}{c}$

$\displaystyle p \cdot c=ab$

2. To get rid of the factor p you have to cancel a multiplication. So use division on both sides of the equation!

$\displaystyle \dfrac{p \cdot c}{p}=\dfrac{ab}{p}$

$\displaystyle \boxed{c=\dfrac{ab}{p}}$

That's all!