# Thread: Solving for a variable

1. ## Solving for a variable

I was hoping to receive some help I'm trying to solve for the variable r.

136.89=65.33 * ((r + 4) * (r + 4) - (4 * 4)) / 12

2. Originally Posted by Henningfeld
I was hoping to receive some help I'm trying to solve for the variable r.

136.89=65.33 * ((r + 4) * (r + 4) - (4 * 4)) / 12

Same way you'd solve for x in

1=2*(x^2+3x+4)/5

Is it easier to see this way?

3. I'm sorry it still has me confused. It has been a few years since I have had to touch an equation like this. I seem to what to start in the wrong directions by multiple by 12 to cancel and then divide by 65.33. In your equation I'm not sure where the 3x comes from. Any suggestions?

4. Originally Posted by Henningfeld
I seem to what to start in the wrong directions by multiple by 12 to cancel and then divide by 65.33.
This is actually in the right direction.

Originally Posted by Henningfeld
In your equation I'm not sure where the 3x comes from.
Expand (r + 4) * (r + 4) - (4 * 4) and you'll see why I put that in there.

5. When I expand that I get (r^2 + 8r + 16) (-16) those two will cancel leaving (r^2 +8r) is this correct?

6. Originally Posted by Henningfeld
When I expand that I get (r^2 + 8r + 16) (-16) those two will cancel leaving (r^2 +8r) is this correct?
Yes, continue.

7. So in a whole equation I have this left 24.95515 = (r^2 + 8r). This is where I begin to second guess what should come next. Do I take the square root of r^2 on both sides then add r to 8r to make 9r then divide by 9?

8. Originally Posted by Henningfeld
So in a whole equation I have this left 24.95515 = (r^2 + 8r). This is where I begin to second guess what should come next. Do I take the square root of r^2 on both sides then add r to 8r to make 9r then divide by 9?
I think you've made a typo somewhere, because 136.89*12/65.33 is approximately 25.1443.

In any case, recall that for ax^2+bx+c=0 we can use the quadratic equation.