Two coils in differential connection have self inductance

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Two coils in differential connection have self inductance of 2 mH and 4 mH and a mutual inductance of 0.15 mH. The equivalent inductance of the combination is _____ mH

1. 1.7 mH
2. 3.0 mH
3. 5.7 mH
4. 1.7 mH

Correct Option: C

When two inductors are connected in series, the effective inductance
L_eff = L₁ + L₂ ± 2M
In this case, L_eff = L₁ + L₂ – 2M
= 2 + 4 – 2 × 0.15 = 5.7 mH.