Calculate two parallel resistors

By Pygmalion
 
Formula:
Rtotal = (R1 * R2) / (R1 + R2)
Derivation:
1/Rtotal = 1/R1 + 1/R2 + ... + 1/Rn
1/Rtotal = 1/R1 + 1/R2
1/Rtotal = 1/R1 * R2/R2 + 1/R2 * R1/R1
1/Rtotal = R2 / R1 * R2 + R1 / R1 * R2
1/Rtotal = (R1 + R2) / (R1 * R2)
Rtotal = (R1 * R2) / (R1 + R2)