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JEE Advanced 2018 · Continuity and Differentiability

Let f:R→Rf:\mathbb{R}\to\mathbb{R} and g:R→Rg:\mathbb{R}\to\mathbb{R} be two non-constant differentiable functions. If f′(x)=(e(f(x)−g(x)))g′(x)f'(x)=(e^{(f(x)-g(x))})g'(x) for all x∈Rx\in\mathbb{R}, and f(1)=g(2)=1f(1)=g(2)=1, then which of the following statement (s) is (are) TRUE?

Answers: B. f(2)>1−log⁡e2f(2)>1-\log_e 2 · C. g(1)>1−log⁡e2g(1)>1-\log_e 2

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Written solution

f′(x)=ef(x)−g(x) g′(x),∀x∈Rf'(x)=e^{f(x)-g(x)}\,g'(x),\quad \forall x\in\mathbb{R}

f(1)=1,g(2)=1f(1)=1,\qquad g(2)=1

Compare f(2), g(1) with 1−log⁡e2\text{Compare } f(2),\ g(1) \text{ with } 1-\log_e 2

f′(x)e−f(x)=g′(x)e−g(x)f'(x)e^{-f(x)}=g'(x)e^{-g(x)}

−ddx(e−f(x))=−ddx(e−g(x))-\dfrac{d}{dx}\left(e^{-f(x)}\right)=-\dfrac{d}{dx}\left(e^{-g(x)}\right)

⇒ddx(e−f(x)−e−g(x))=0\Rightarrow \dfrac{d}{dx}\left(e^{-f(x)}-e^{-g(x)}\right)=0

⇒e−f(x)−e−g(x)=C (constant)\Rightarrow e^{-f(x)}-e^{-g(x)}=C\ (\text{constant})

x=1: C=e−1−e−g(1)x=1:\ C=e^{-1}-e^{-g(1)}

x=2: C=e−f(2)−e−1x=2:\ C=e^{-f(2)}-e^{-1}

e−1−e−g(1)=e−f(2)−e−1e^{-1}-e^{-g(1)}=e^{-f(2)}-e^{-1}

⇒e−f(2)+e−g(1)=2e−1\Rightarrow e^{-f(2)}+e^{-g(1)}=2e^{-1}

e−g(1)>0⇒e−f(2)<2e−1e^{-g(1)}>0 \Rightarrow e^{-f(2)}<2e^{-1}

⇒−f(2)<ln⁡2−1⇒f(2)>1−log⁡e2\Rightarrow -f(2)<\ln 2-1 \Rightarrow f(2)>1-\log_e 2

⇒(2) true, (1) false\Rightarrow (2) \text{ true},\ (1) \text{ false}

e−f(2)>0⇒e−g(1)<2e−1e^{-f(2)}>0 \Rightarrow e^{-g(1)}<2e^{-1}

⇒g(1)>1−log⁡e2⇒(3) true, (4) false\Rightarrow g(1)>1-\log_e 2 \Rightarrow (3) \text{ true},\ (4) \text{ false}

True statements: (2) and (3)\text{True statements: } (2) \text{ and } (3)

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