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Verify Rolle'S Theorem for the Following Function on the Indicated Interval F(X) = Sin X + Cos X on [0, π/2] ? - Mathematics

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Question

Verify Rolle's theorem for the following function on the indicated interval f(x) = sin x + cos x on [0, π/2] ?

Sum

Solution

 The given function is \[f\left( x \right) = \sin x + \cos x\] .

Since \[\sin x \text { and } \cos x\] are everywhere continuous and differentiable, \[f\left( x \right) = \sin x + \cos x\] is continuous on 

\[\left[ 0, \frac{\pi}{2} \right]\] and differentiable on \[\left( 0, \frac{\pi}{2} \right)\] .
Also,
\[f\left( \frac{\pi}{2} \right) = f\left( 0 \right) = 1\]
Thus,\[f\left( x \right)\] satisfies all the conditions of Rolle's theorem.
Now, we have to show that there exists
\[c \in \left( 0, \frac{\pi}{2} \right)\] such that  \[f'\left( c \right) = 0\] .
We have

\[f\left( x \right) = \sin x + \cos x\]

\[ \Rightarrow f'\left( x \right) = \cos x - \sin x\]

\[\therefore f'\left( x \right) = 0\]
\[ \Rightarrow \cos x - \sin x = 0\]
\[ \Rightarrow \tan x = 1\]
\[ \Rightarrow x = \frac{\pi}{4}\]

Thus,\[c = \frac{\pi}{4} \in \left( 0, \frac{\pi}{2} \right)\] such that

\[f'\left( c \right) = 0\] .

​Hence, Rolle's theorem is verified.
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Chapter 15: Mean Value Theorems - Exercise 15.1 [Page 9]

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RD Sharma Mathematics [English] Class 12
Chapter 15 Mean Value Theorems
Exercise 15.1 | Q 3.11 | Page 9

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