Star-Delta Transformation

Delta → star:

$R_a = \frac{R_{ab} R_{ca}}{R_{ab} + R_{bc} + R_{ca}}$

(the product of the two delta resistors touching that node, over the sum of all three).

Star → delta:

$R_{ab} = \frac{R_aR_b + R_bR_c + R_cR_a}{R_c}$

For three equal resistors: $R_Y = R_\Delta / 3$. This transformation solves networks such as the unbalanced Wheatstone bridge that have no simple series or parallel combinations.