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JEE Advanced 2006
Exam
JEE Advanced
Year
2006
Subject
Mathematics
Chapter
Application of Derivatives

JEE Advanced 2006 · Mathematics · Application of Derivatives

If f(x) is twice differentiable function such that f(a)=0, f(b)=2,...

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

Correct answer

6

Step-by-step solution

Let, g(x)=ddx[f(x)\cdotf′(x)]g(x)=\frac{d}{d x} \left[f(x) \cdotf^{\prime}(x)\right] to get the zero of g(x)g(x) we take function h(x)=f(x)\cdotf′(x)h(x)=f(x) \cdotf^{\prime}(x) between any two roots of h(x)h(x) there lies atleast one root of h′(x)=0h^{\prime}(x)=0 ⇒g(x)=0⇒h(x)=0⇒f(x)=0 or f′(x)=0 If f(x)=0 has 4 minimum solutions, f′(x)=0 has 3 minimum solutions, h(x)=0 has 7 minimum solutions, then h′(x)=g(x)=0 has 6 minimum solutions. \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}
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