why in Q11(b) HC verma to calculate minmum force needed we did the diffirntiation of the denominator as in the - Physics - Work Energy and Power

Question

why in Q11(b) HC verma to calculate minmum force needed we did the
diffirntiation of the denominator as in the solutions? Plz give me
the reason for that

Pintu B. · Accepted Answer

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why in Q11(b) HC verma to calculate minmum force needed we did the diffirntiation of the denominator as in the solutions? Plz give me the reason for that ..