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Three charges +Q, q, +Q are placed respectively, at distance, 0, d/2 and d from the origin, on the x-axis. If the net force experienced by +Q, placed at x = 0, is zero, then value of q is :

1. +Q/2
2. –Q/4
3. +Q/4
4. –Q/2

Correct answer: –Q/4

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