Solving for a variable

• Sep 1st 2010, 11:34 AM
Henningfeld
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

• Sep 1st 2010, 12:04 PM
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Quote:

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?
• Sep 1st 2010, 12:09 PM
Henningfeld
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?
• Sep 1st 2010, 12:13 PM
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Quote:

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.

Quote:

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.
• Sep 1st 2010, 12:21 PM
Henningfeld
When I expand that I get (r^2 + 8r + 16) (-16) those two will cancel leaving (r^2 +8r) is this correct?
• Sep 1st 2010, 12:24 PM
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Quote:

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.
• Sep 1st 2010, 12:29 PM
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?
• Sep 1st 2010, 12:36 PM
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Quote:

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.