JEE Advance - Mathematics (2016 - Paper 2 Offline - No. 6)

Let f: R $$ \to \left( {0,\infty } \right)$$ and g : R $$ \to $$ R be twice differentiable functions such that f'' and g'' are continuous functions on R. Suppose f'$$(2)$$ $$=$$ g$$(2)=0$$, f''$$(2)$$$$ \ne 0$$ and g'$$(2)$$ $$ \ne 0$$. If
$$\mathop {\lim }\limits_{x \to 2} {{f\left( x \right)g\left( x \right)} \over {f'\left( x \right)g'\left( x \right)}} = 1,$$ then
$$f$$ has a local minimum at $$x=2$$
$$f$$ has a local maximum at $$x=2$$
$$f''(2)>f(2)$$
$$f(x)-f''(x)=0$$ for at least one $$x \in R$$

Explanation

Let f : R $$\to$$ (0, $$\infty$$) and g : R $$\to$$ R.

f(x) > 0 $$\forall$$x $$\in$$ R

It is given that f'(2) = 0, g(2) = 0, f''(2) $$\ne$$ 0 and g'(2) $$\ne$$ 0.

It is also given that

$$\mathop {\lim }\limits_{x \to 2} {{f(x)g(x)} \over {f'(x)g'(x)}} = 1$$ $$\left( {{0 \over 0}} \right)$$

Applying L' Hospital rule, we get

$$\mathop {\lim }\limits_{x \to 2} {{f'(x)g(x) + g'(x)f(x)} \over {f''(x)g'(x) + f'(x)g''(x)}} = 1$$

For finite limit, we get

$${{f'(2)g(2) + g'(2)f(2)} \over {f''(2)g'(2) + f'(2)g''(2)}} = 1$$

$${{g'(2)f(2)} \over {f''(2)g'(2)}} = 1$$

$${{f(2)} \over {f''(2)}} = 1 \Rightarrow f''(2) = f(2) > 0$$ and f'(2) = 0 which means that f(x) has local minima at x = 2.

Hence, option (A) is correct.

$$f(2) - f''(2) = 0$$

Therefore, we can say that $$f(x) - f''(x) = 0$$ has at least one solution in x $$\in$$ R.

Hence, option (D) is correct.

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