with a little algebraic manipulation we can solve for pi...
d-c / pi
we want to get pi by itself so first we multiply both sides by pi to get it out of the denominator...
pi × (d-c) = pi / pi
pi × (d-c) = 1
Now get pi by itself by dividing both sides by d-c...
pi × (d-c) / d-c = 1 / d-c
simplify
pi × 1 = 1 / d-c
pi = 1 / d-c
Done!