Expert: Abe Mantell - 8/28/2010

Question
Hi,

I was doing a HW problem (for my electronics class) and it
comes down that I have the following equation below, and I'm
left with just the math part, needing to figure out what R
equals to and somehow I'm having
some difficulty doing the math oddly. Can you help? Thanks!

10[R/(1k + R)] - 10[1.5k + R)] = 1V

k means 1000 (like in kilowatts vs watts) and V is voltage.

Answer
Hello Mike,

Let me rewrite your equation:
10[R/(1000 + R)] - 10[1500 + R] = 1

Now getting rid of the brackets:
10R/(1000+R) - 15000 - 10R = 1

Multiplying by (1000+R) gives:
10R - 15000(1000+R) - 10R(1000+R) = 1000+R

Distributing gives:
10R-15000000-15000R-10000R-10R^2=1000+R

Simplifying:
10R^2 + 24991R + 15001000 = 0

Using the quadratic formula, we get
R = -1497.088285 OR -1002.011715

Are you sure your equation is correct?

Abe