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JEE Advanced 2017 · Application of Derivatives

If f:R→Rf : \mathbb{R} \to \mathbb{R} is a differentiable function such that f′(x)>2f(x)f'(x) > 2f(x) for all x∈Rx \in \mathbb{R}, and f(0)=1f(0) = 1, then

Answers: A. f(x)f(x) is increasing in (0,∞)(0, \infty) · C. f(x)>e2xf(x) > e^{2x} in (0,∞)(0, \infty)

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Written solution

Given: f′(x)>2f(x)  ∀x∈R\text{Given: } f'(x) > 2f(x)\ \ \forall x \in \mathbb{R}

f(0)=1f(0) = 1

To find: the correct statement among (1)–(4)\text{To find: the correct statement among (1)--(4)}

Let g(x)=f(x) e−2x\text{Let } g(x) = f(x)\,e^{-2x}

g′(x)=e−2x(f′(x)−2f(x))>0g'(x) = e^{-2x}\big(f'(x) - 2f(x)\big) > 0

g strictly increasing,g(0)=f(0)e0=1g \text{ strictly increasing}, \qquad g(0) = f(0)e^{0} = 1

x>0⇒g(x)>g(0)=1x > 0 \Rightarrow g(x) > g(0) = 1

⇒f(x)e−2x>1⇒f(x)>e2x\Rightarrow f(x)e^{-2x} > 1 \Rightarrow f(x) > e^{2x}

⇒(3) TRUE\Rightarrow (3)\ \text{TRUE}

f(x)>e2x>0⇒f′(x)>2f(x)>0⇒(1) TRUEf(x) > e^{2x} > 0 \Rightarrow f'(x) > 2f(x) > 0 \Rightarrow (1)\ \text{TRUE}

f′(x)>2f(x)>2e2x>e2x⇒(4) falsef'(x) > 2f(x) > 2e^{2x} > e^{2x} \Rightarrow (4)\ \text{false}

f increasing⇒(2) falsef \text{ increasing} \Rightarrow (2)\ \text{false}

TRUE: (3) f(x)>e2x, and (1) f increasing\text{TRUE: } (3)\ f(x) > e^{2x}, \text{ and } (1)\ f \text{ increasing}

(1) and (3)\boxed{(1)\ \text{and}\ (3)}

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