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Verify Lagrange'S Mean Value Theorem for the Following Function on the Indicated Intervals. Find a Point 'C' in the Indicated Interval as Stated by the Lagrange'S Mean F(X) = X3 − 5x2 − 3x on [1, 3] ?

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Question

Verify Lagrange's mean value theorem for the following function on the indicated intervals. find a point 'c' in the indicated interval as stated by the Lagrange's mean value theorem f(x) = x3 − 5x2 − 3x on [1, 3] ?

Sum
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Solution

We have,

\[f\left( x \right) = x^3 - 5 x^2 - 3x\]

Since polynomial function is everywhere continuous and differentiable
Therefore, \[f\left( x \right)\] is continuous on \[\left[ 1, 3 \right]\] and differentiable on \[\left( 1, 3 \right)\]

Thus, both the conditions of lagrange's theorem are satisfied.
Consequently, there exists some 

\[c \in \left( 1, 3 \right)\] such that
\[f'\left( c \right) = \frac{f\left( 3 \right) - f\left( 1 \right)}{3 - 1} = \frac{f\left( 3 \right) - f\left( 1 \right)}{2}\]
Now, \[f\left( x \right) = x^3 - 5 x^2 - 3x\]
\[f'\left( x \right) = 3 x^2 - 10x - 3\],\[f\left( 3 \right) = - 27\],\[f\left( 1 \right) = - 7\]

∴  \[f'\left( x \right) = \frac{f\left( 3 \right) - f\left( 1 \right)}{2}\]

\[\Rightarrow 3 x^2 - 10x - 3 = \frac{- 20}{2}\]

\[ \Rightarrow 3 x^2 - 10x + 7 = 0\]

\[ \Rightarrow x = 1, \frac{7}{3}\]

Thus, \[c = \frac{7}{3} \in \left( 1, 3 \right)\] such that \[f'\left( c \right) = \frac{f\left( 3 \right) - f\left( 1 \right)}{3 - 1}\] .

Hence, Lagrange's theorem is verified.

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Chapter 14: Mean Value Theorems - Exercise 15.2 [Page 17]

APPEARS IN

R.D. Sharma Mathematics Volume 1 and 2 [English] Class 12
Chapter 14 Mean Value Theorems
Exercise 15.2 | Q 1.16 | Page 17

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