Q74 / 120 · JEE Advanced 2006 · Mathematics

If $f(x)$ is twice differentiable function such that $f(a)=0, f(b)=2, f(c)=1, f(d)=2$, $f(e)=0$, where $a < b < c < d < e$, then the minimum number of zeros of $g(x)=\left\{f^{\prime}(x)\right\}^2+f^{\prime \prime}(x) \cdot f(x)$ in the interval $[a, e]$ is

Correct answer: 6

Solution

Let, $g(x)=\frac{d}{d x}\left[f(x) \cdot f^{\prime}(x)\right]$ to get the zero of $g(x)$ we take function $h(x)=f(x) \cdot f^{\prime}(x)$ between any two roots of $h(x)$ there lies atleast one root of $h^{\prime}(x)=0$ $$ \begin{aligned} & \Rightarrow \quad g(x)=0 \\ & \Rightarrow \quad h(x)=0 \\ & \Rightarrow \quad f(x)=0 \text { or } f^{\prime}(x)=0 \\ & \text { If } f(x)=0 \text { has } 4 \text { minimum solutions, } \\ & f^{\prime}(x)=0 \text { has } 3 \text { minimum solutions, } \\ & h(x)=0 \text { has } 7 \text { minimum solutions, then } \\ & h^{\prime}(x)=g(x)=0 \text { has } 6 \text { minimum solutions. } \\ & \end{aligned} $$

Hence, the correct answer is 6.

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