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Verify Rolle'S Theorem for the Following Function on the Indicated Interval F (X) = X2 + 5x + 6 on the Interval [−3, −2] ?

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प्रश्न

Verify Rolle's theorem for the following function on the indicated interval f (x) = x2 + 5x + 6 on the interval [−3, −2]  ?

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उत्तर

Given function is \[f\left( x \right) = x^2 + 5x + 6\] .

We know that a polynomial function is everywhere derivable and hence continuous.
So, being a polynomial function, 

\[f\left( x \right)\] is continuous and derivable on \[\left[ - 3, - 2 \right]\] .

Also,

\[f\left( - 3 \right) = \left( - 3 \right)^2 + 5\left( - 3 \right) + 6 = 9 - 15 + 6 = 0\]

\[f\left( - 2 \right) = \left( - 2 \right)^2 + 5\left( - 2 \right) + 6 = 4 - 10 + 6 = 0\]

\[ \therefore f\left( - 3 \right) = f\left( - 2 \right) = 0\]

Thus, all the conditions of the Rolle's theorem are satisfied.
Now, we have to show that there exists \[c \in \left[ - 3, - 2 \right]\] such that \[f'\left( c \right) = 0\] .

We have

\[f\left( x \right) = x^2 + 5x + 6\]

\[ \Rightarrow f'\left( x \right) = 2x + 5\]

\[ \therefore f'\left( x \right) = 0 \Rightarrow 2x + 5 = 0\]

\[ \Rightarrow x = \frac{- 5}{2}\]

Thus, 

\[c = \frac{- 5}{2} \in \left( - 3, - 2 \right) \text { such that } f'\left( c \right) = 0\] .
Hence, Rolle's theorem is verified.
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अध्याय 14: Mean Value Theorems - Exercise 15.1 [पृष्ठ ९]

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आर.डी. शर्मा Mathematics Volume 1 and 2 [English] Class 12
अध्याय 14 Mean Value Theorems
Exercise 15.1 | Q 2.8 | पृष्ठ ९

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