1/ (2pi FC) - is the capacitive reactance (equivalent resistance) of a capacitor at a frequency F. Replace C with L for inductive reactance.

Small typo there, so just for completness sake: For inductive reactance the formula is the reciprocal (with regards to capacative reactance); X

_{L} = 2pi FL.

Since nobody have mentioned it, calculating resistance of resistors in parallel is also handy:

R = R1*R2/(R1+R2)

Personally I never use the above form, I like the reciprocals variant as it is simpler with more than 2 resistors:

1/R = 1/R1 + 1/R2 ... + 1/Rn

Same goes for capacitors in series btw:

1/C = 1/C1 + 1/C2 ... + 1/Cn

While capacitors in parallel just adds up (like resistors in series, so they are kind of opposite)

C = C1 + C2 ... +Cn