Resistors can be connected together in two ways to give different overall values. This is especially useful if you do not have a resistor of the correct value and need to make it up from other available ones.
1. Resistors in SERIES - When resistors are connected in series, their values are added together:
For example: 1K+1K+3K9=5K9 (total value)
2. Resistors in PARALLEL -When resistors are connected in parallel, their total resistance is given as:
1/Rtotal = 1/R1 + 1/R2
For example: 1/Rtotal = 1/1K + 1/1K = 0.5K or 500 ohms