Engineering
Mathematics
Concept of Derivative
Question

Let  f  be a differentiable function such that  f (1) = 2 and  f ' (x) = f (x) for all x∈R. If  h (x) = f(f(x)), then h'(1)  is equal to

2e

2e2

4e

4e2

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Solution

f '(x) = f(x) ⇒ dydx  = y ⇒ ln y = x + c

∴ y = λex ⇒ f(1) = 2

∴ 2 = λe ⇒ λ = 2e

∴y=2exe

∴h(x)=f(f(x))=f(2exe)=2e2exee

∴h'(x)=2e2exee · 2exe=4e2 · e2exe.  ex

∴ h'(1) = 4e2  · e2 · e = 4e.