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

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प्रश्न

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

बेरीज
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उत्तर

The given function is \[f\left( x \right) = \cos2x\] .

Since \[\cos2 \ x\] is everywhere continuous and differentiable. 

Therefore, \[f\left( x \right)\]  is continuous on  \[\left[ 0, \pi \right]\] and differentiable on \[\left( 0, \pi \right)\] .

Also,\[f\left( \pi \right) = f\left( 0 \right) = 1\]

Thus, 
\[f\left( x \right)\] satisfies all the conditions of Rolle's theorem. 
Now, we have to show that there exists \[c \in \left( 0, \pi \right)\] such that \[f'\left( c \right) = 0\] .
We have
\[f\left( x \right) = \cos2x\]
\[ \Rightarrow f'\left( x \right) = - 2\sin2x\]
\[\therefore f'\left( x \right) = 0\]
\[ \Rightarrow - 2\sin2x = 0\]
\[ \Rightarrow \sin2x = 0\]
\[ \Rightarrow 2x = \pi\]
\[ \Rightarrow x = \frac{\pi}{2}\]
Thus,
\[c = \frac{\pi}{2} \in \left( 0, \pi \right)\] such that  \[f'\left( c \right) = 0\] .
​Hence, Rolle's theorem is verified.
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पाठ 15: Mean Value Theorems - Exercise 15.1 [पृष्ठ ९]

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आरडी शर्मा Mathematics [English] Class 12
पाठ 15 Mean Value Theorems
Exercise 15.1 | Q 3.06 | पृष्ठ ९

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