To apply simple series–parallel circuit technique for the solution of network, something is required to transform the resistors connected in delta to star or vice versa. At such places, just to simplify the network delta–star or star–delta transformations applied.
2.9.1 Delta–Star Transformation
Consider a circuit shown in Figure 2.67(a), where three resistors RAB, RBC, and RCA are connected in delta. This circuit is converted to a star-connection circuit, as shown in Figure 2.67(b). Let the resistance be RA, RB, and RC.
Fig. 2.67 (a) Delta connections (b) Equivalent star connection
For the two circuits to be equivalent, the resistance measured between any two of the terminals A, B, and C must be the same in the two cases.
RAB for star connections = RAB for delta connections.
Similarly,
and
Subtracting Equation (2.62) from Equation (2.61) and adding the result to Equation (2.63), we get
To remember this relation, we should refer to Figure 2.68. According to Equations (2.64), (2.65), and (2.66),
Resistance of any arm of 
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