Question

# Let f$$:\mathbb{R}\rightarrow \mathbb{R}$$ and $$g:\mathbb{R}\rightarrow \mathbb{R}$$ be two non-constant differentiable functions. If $$f'(x)=(e^{(f(x)-g(x))})g'(x)$$ for all $$x\epsilon \mathbb{R}$$, and $$f(1)=g(2)=1$$, then which of the following statement(s) is (are) TRUE?

A
f(2)<1loge2
B
f(2)>1loge2
C
g(1)>1loge2
D
g(1)<1loge2

Solution

## The correct options are B $$g(1) > 1 -log_e 2$$ D $$f(2) > 1-log_e 2$$$$f'(x)=e^{(f(x)-g(x))}g'(x)\forall x\epsilon \mathbb{R}$$$$\Rightarrow e^{-f(x)}\cdot f'(x)-e^{-g(x)}g'(x)=0$$$$\Rightarrow \displaystyle\int(e^{-f(x)}f'(x)-e^{-g(x)}\cdot g'(x))dx=C$$$$\Rightarrow -e^{-f(x)}+e^{-g(x)}=C$$$$\Rightarrow -e^{-f(1)}+e^{-g(1)}=-e^{-f(2)}+e^{-g(2)}$$$$\Rightarrow -\displaystyle\dfrac{1}{e}+e^{-g(1)}=-e^{-f(2)}+\dfrac{1}{e}$$$$\Rightarrow e^{-f(2)}+e^{-g(1)}=\dfrac{2}{e}$$$$\therefore$$ $$e^{-f(2)} < \dfrac{2}{e}$$ and $$e^{-g(1)} < \dfrac{2}{e}$$$$\Rightarrow -f(2) < log_{e}2 -1$$ and $$-g(1) < log_{e}2-1$$$$\Rightarrow f(2) > 1 -log_{e}2$$  and  $$g(1) > 1-log_{e}2$$.Mathematics

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