Magnetic field due to an uneven current loop

[hiigaranace]

Homework Statement

"A closed circuit consists of two semicircles of radii 40cm and 20cm that are connected by straight segments. A current of 3.0 A exists in this circuit and has a clockwise direction. Find the magnetic field at point P (center of the figures).

Homework Equations

B = (mu * I)/(2piR)

The Attempt at a Solution

I had thought adapting the above equation would be the way to go, just using two different radii. I ended up with:

$$\frac{1}{2}$$$$\mu$$$$\frac{I}{2\pi}$$ * $$\left(\frac{1}{20} - \frac{1}{40}\right)$$

LaTeX didn't like the above, so in plaintext:

(1/2)(mu0 * I)/(2pi) * (1/20-1/40)

This didn't work, though, and I'm not entirely sure why.

 

[ideasrule]

That's the equation for an infinitely long, straight wire. You have two small loops, not two infinitely long wires.