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5. Continuity and Differentiation
normal
Let $f: R \rightarrow R$ be a differentiable function such that $f(a)=0=f(b)$ and $f^{\prime}(a) f^{\prime}(b) > 0$ for some $a < b$. Then, the minimum number of roots of $f^{\prime}(x)=0$ in the interval $(a, b)$ is
A
$3$
B
$2$
C
$1$
D
$0$
(KVPY-2010)
Solution
(b)
Given,
$f: R \rightarrow R$ be a differentiable function in $(a, b)$
and $\quad f(a)=f(b)=0, a < b$
and $f^{\prime}(a) f^{\prime}(b) > 0$
Since, $f^{\prime}(a) f^{\prime}(b) > 0$.
$\therefore f^{\prime}(a)$ and $f^{\prime}(b)$ both are positive or negative when both are positive then at least one root. $f^{\prime}(x)=0$ lie in interval, also when both are negative then at least one root lie in the interval.
[by Rolle's theorem]
$\therefore$ Minimum number of roots are $2$.
Standard 12
Mathematics