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## Homework Statement

Express:

in terms of R, L(t), l, and φ

## Homework Equations

Law of Sines and Cosines

## The Attempt at a Solution

I have used:

c^2=a^2+b^2-2ab*cosφ which resulted in

r^2=R^2+(L(t)-l)^2-2*R*(L(t)-l)*cosφ

and used the law of sines to get

R*sinφ=r*sinθ

r=(R*sinφ)/sinθ

I'm struggling figuring out where to go from here. I know I'm looking to replace the dl x r portion of the Biot-Savart law. If anyone could help prod me in the right direction I'd appreciate it. Thanks