Question
There exists a function $$f\left( x \right)$$ satisfying $$f\left( 0 \right) = 1,\,f'\left( 0 \right) - 1,\,f\left( x \right) > 0$$ for all $$x$$ and
A.
$$f'\left( x \right) < 0$$ for all $$x$$
B.
$$ - 1 < f''\left( x \right) < 0$$ for all $$x$$
C.
$$ - 2 \leqslant f''\left( x \right) \leqslant - 1$$ for all $$x$$
D.
$$f''\left( x \right) \leqslant - 2$$ for all $$x$$
Answer :
$$f'\left( x \right) < 0$$ for all $$x$$
Solution :
$$f\left( x \right) = {e^{ - x}}$$ satisfies $$f\left( 0 \right) = 1,\,f'\left( 0 \right) = - 1,\,f\left( x \right) > 0$$
It also satisfies $$f'\left( x \right) < 0$$ for all $$x.$$