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Write the Value of B for Which F ( X ) = { 5 X − 4 0 < X ≤ 1 4 X 2 + 3 B X 1 < X < 2 is Continuous at X = 1. - Mathematics

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Question

Write the value of b for which \[f\left( x \right) = \begin{cases}5x - 4 & 0 < x \leq 1 \\ 4 x^2 + 3bx & 1 < x < 2\end{cases}\]  is continuous at x = 1.

 

Sum
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Solution

Given,  

\[f\left( x \right) = \begin{cases}5x - 4 & 0 < x \leq 1 \\ 4 x^2 + 3bx & 1 < x < 2\end{cases}\]

If  \[f\left( x \right)\]  is continuous at \[x = 1\] , then

\[\lim_{x \to 1^-} f\left( x \right) = \lim_{x \to 1^+} f\left( x \right) = f\left( 1 \right)\]            ...(1)
Now, 
\[\lim_{x \to 1^-} f\left( x \right) = \lim_{h \to 0} f\left( 1 - h \right) = \lim_{h \to 0} 5\left( 1 - h \right) - 4 = 5 - 4 = 1\]
\[\lim_{x \to 1^+} f\left( x \right) = \lim_{h \to 0} f\left( 1 + h \right) = \lim_{h \to 0} 4 \left( 1 + h \right)^2 + 3b\left( 1 + h \right) = 4 + 3b\]
 
Also,
\[f\left( 1 \right) = 5\left( 1 \right) - 4 = 1\]

\[\lim_{x \to 1^-} f\left( x \right) = \lim_{x \to 1^+} f\left( x \right) = f\left( 1 \right) \left[ \text{ From eq } . (1) \right]\]

\[ \Rightarrow 1 = 4 + 3b = 1\]

\[\Rightarrow 1 = 4 + 3b\]

\[ \Rightarrow - 3 = 3b\]

\[ \Rightarrow b = - 1\]

Thus, for 

\[b = - 1\] , the function 
\[f\left( x \right)\] is continuous at  \[x = 1\] .
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Chapter 9: Continuity - Exercise 9.3 [Page 42]

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RD Sharma Mathematics [English] Class 12
Chapter 9 Continuity
Exercise 9.3 | Q 10 | Page 42

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