If and for all

MATHEMATICS · Single correct · A Level · Mathematics · Differentiation · Differentiation

Problem

If f′(1)=1f'(1)=1 and ddx(f(2x))=f′(x)\frac{d}{dx}\bigl(f(2x)\bigr)=f'(x) for all x>0x>0, and if f′(x)f'(x) is differentiable, then there exists a number c∈(2,4)c\in(2,4) such that f′′(c)f''(c) is equal to (1) −14-\frac{1}{4} (2) −18-\frac{1}{8} (3) 14\frac{1}{4} (4) 18\frac{1}{8}

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