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CBSE Class 12 Math 2024 All Sets Solved Paper

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Question : 4 of 20
Marks: +1, -0
The number of points of discontinuity of f(x)={x+3,if x32x,if 3<x<36x+2,if x3f(x) = \begin{cases} |x| + 3, & \text{if } x \le -3 \\ -2x, & \text{if } -3 < x < 3 \\ 6x + 2, & \text{if } x \ge 3 \end{cases} is
Solution:  
Given f(x)={x+3,if x32x,if 3<x<36x+2,if x3f(x) = \begin{cases} |x| + 3, & \text{if } x \le -3 \\ -2x, & \text{if } -3 < x < 3 \\ 6x + 2, & \text{if } x \ge 3 \end{cases}
When x = -3
If f(x) is continuous for x = -3 then
limx3f(x)=limx3+f(x)=f(3)\lim\limits_{x\rightarrow 3^{-}}f(x)=\lim\limits_{x\rightarrow -3^{+}}f(x)=f(-3)
Finding L.H.L.
limx3x+3\lim\limits_{x\rightarrow 3^{-}}|x|+3
=limh03h+3= \lim\limits_{h\rightarrow 0}|-3-h|+3
Putting h = 0 then we get,
= | -3 - 0 | + 3
= | -3 | + 3
= 6
Finding R.H.L.
limx3+2x=limh02(3+h)\lim\limits_{x\rightarrow -3^{+}}-2x=\lim\limits_{h\rightarrow 0}-2(-3+h)
=limh062h= \lim\limits_{h\rightarrow 0}6-2h
Putting h = 0 then we get,
= 6 - 2 ×\times 0
= 6
Find f(x) at x = -3
f(-3) = | -3 | + 3 = 6
Hence, limx3f(x)=limx3+f(x)=f(3)\lim\limits_{x\rightarrow 3^{-}}f(x)=\lim\limits_{x\rightarrow -3^{+}}f(x)=f(-3)
Therefore, the function f(x) is continuous at x = -3.
When x = 3
If f(x) is continuous for x = 3 then
limx3f(x)=limx3+f(x)=f(3)\lim\limits_{x\rightarrow 3^{-}}f(x)=\lim\limits_{x\rightarrow -3^{+}}f(x)=f(3)
Finding L.H.L.
limx32x=limh02(3h)\lim\limits_{x\rightarrow 3^{-}}-2x=\lim\limits_{h\rightarrow 0}-2(-3-h)
=limh06+2h= \lim\limits_{h\rightarrow 0}-6+2h
Putting h=0 then we get,
= -6 + 2 ×\times 0
= -6
Finding R.H.L.
limx3+6x+2=limh06(3+h)+2\lim\limits_{x\rightarrow -3^{+}}6x+2=\lim\limits_{h\rightarrow 0}6(3+h)+2
=limh018+6h+2= \lim\limits_{h\rightarrow 0}18+6h+2
Putting h = 0 then we get,
= 18 + 6 ×\times 0 + 2
= 20
Find f(x) at x = 3
f(3) = 6x + 2 at x = 3
f(3) = 6 ×\times 3 + 2 = 20
Hence, limx3f(x)limx3+f(x)=f(3)\lim\limits_{x\rightarrow 3^{-}}f(x) \ne \lim\limits_{x\rightarrow -3^{+}}f(x)=f(-3)
Therefore, the function
f(x) is discontinuous at x = 3
When x < -3,
For x < -3, f(x) = | x | + 3
Since the function f(x) = | x | + 3 is a modulus function so it is continuous.
∴ f(x) is continuous for x < -3
When x > 3,
For x > 3, f(x) = 6x + 2
Since the function f(x) = 6x + 2 is a polynomial so it is continuous.
∴ f(x) is continuous for x > 3
When -3 < x < 3
For -3 < x < 3, f(x) = -2x
Since the function f(x) = -2x is a polynomial so it is continuous.
∴ f(x) is continuous for -3 < x < 3
f(x)={x+3,if x32x,if 3<x<36x+2,if x3f(x) = \begin{cases} |x| + 3, & \text{if } x \le -3 \\ -2x, & \text{if } -3 < x < 3 \\ 6x + 2, & \text{if } x \ge 3 \end{cases} discontinuous at x = 3 only.
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