If the function \(\rm {f}(\mathbf{x})=\left\{\begin{array}{cc} 1, & x \leq 2 \\ a x+b, & 2<x<4 \\ 7, & x \geq 4 \end{array}\right.\) is continuous at x = 2 and 4, then the values of a and b are.

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  1. a = 3, b = −5 
  2. a = −5, b = 3
  3. a = −3, b = 5
  4. a = 5, b = −3 

Answer (Detailed Solution Below)

Option 1 : a = 3, b = −5 
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Detailed Solution

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Calculation 

Since f(x) is continuous at x=2

∴ f(2) = \(\lim_{x\rightarrow2}\) f(x) = 1 = \(\lim_{x\rightarrow2}\) (ax+b)

∴ 1 = 2a + b ... (i)

Again f(x) is continuous at x=4,

∴ f(4) = \(\lim_{x\rightarrow4}\) f(x) = 7 = \(\lim_{x\rightarrow4}\) (ax+b)

∴ 7 = 4a + b ... (ii)

Solving (i) and (ii), we get a = 3, b = -5

Hence option 1 is correct

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