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Question

Find the value of k for which the expression \(x^3 - 3x^2 - 7x - k\) will be divisible by \((x - 5)\).

This question was previously asked in
RRB NTPC 2025 Under Graduate CBT 1 Question Paper PDF (20-Jun-2026) (Shift 3)
The correct answer is

15

By the remainder theorem, a polynomial \(f(x)\) is divisible by \((x - 5)\) exactly when \(f(5) = 0\).

Here \(f(x) = x^3 - 3x^2 - 7x - k\), so substitute x = 5.

Compute each term: \(5^3 = 125\), \(3 \times 5^2 = 75\), and \(7 \times 5 = 35\).

So \(f(5) = 125 - 75 - 35 - k = 15 - k\).

Set this equal to zero: \(15 - k = 0\), giving \(k = 15\).

Hence, the required value of k is 15.

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