Calculate the equivalent resistance of the following combinations of resistors r1, r2, r3, r4.
Since r3 and r4 are in parallel
∴ Equivalent resistance R1 of this combination is given by
1/R1 = 1/r3 + 1/r4 = r4 + r3/r3r4
Or R1 = r3r4/r3 + r4
Now r1, r2 and R1 are in series
∴ Equivalent resistance R of the whole combination is
R = r1 + r2 + r3r4/r3 + r4 = (r1 + r2)(r3 + r4) + r3r4/r3 + r4 Ω