Question 44613
u x i/2pi x (D+d)+ u x i/2pi x (D-d) is equalt to u x i x D/pi(D sq -d sq) 

ui/(2pi(D+d)) + ui/(2pi(D-d))

LCD = 2pi(D^2-d^2)

Write each fraction over the LCD, as follows:

=ui(D-d)/LCD + ui(D+d)/LCD

=Add the numerators and write the sum over the LCD, as follows:

=[uiD-uid+uiD+uid]/LCD

=2uiD/[2pi(D^2-d^2)

= uiD/[(pi)(D^2-d^2)

I am assuming your pi is not p*i but rather the number pi=3.1459...

Cheers,
Stan H.