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Q7:
Find all points of discontinuity of $f$, where $f$ is defined by $f(x) = \begin{cases} |x| + 3, & \text{if } x \le -3 \\ -2x, & \text{if } -3 < x < 3 \\ 6x + 2, & \text{if } x \ge 3 \end{cases}$
Solution :
Given Variables & Function Definition
The function $f(x)$ is defined piecewise over the domain $\mathbb{R}$:
- $f(x) = |x| + 3$ for $x \le -3$
- $f(x) = -2x$ for $-3 < x < 3$
- $f(x) = 6x + 2$ for $x \ge 3$
Since the function definition changes at the critical points $x = -3$ and $x = 3$, we must analyze the continuity at these points individually. For all other intervals ($x < -3$, $-3 < x < 3$, and $x > 3$), $f(x)$ is a polynomial or a simple absolute value function, which are inherently continuous in their respective domains [Per Theorem: Polynomial functions are continuous everywhere].
Step 1: Analyzing Continuity at $x = -3$
For $f(x)$ to be continuous at $x = -3$, the following condition must hold: $\lim_{x \to -3^-} f(x) = \lim_{x \to -3^+} f(x) = f(-3)$.
1. Left-hand limit (LHL): $\lim_{x \to -3^-} (|x| + 3) = |-3| + 3 = 3 + 3 = 6$.
2. Right-hand limit (RHL): $\lim_{x \to -3^+} (-2x) = -2(-3) = 6$.
3. Functional value: $f(-3) = |-3| + 3 = 6$.
Since $\text{LHL} = \text{RHL} = f(-3) = 6$, the function is continuous at $x = -3$.
Step 2: Analyzing Continuity at $x = 3$
For $f(x)$ to be continuous at $x = 3$, the following condition must hold: $\lim_{x \to 3^-} f(x) = \lim_{x \to 3^+} f(x) = f(3)$.
1. Left-hand limit (LHL): $\lim_{x \to 3^-} (-2x) = -2(3) = -6$.
2. Right-hand limit (RHL): $\lim_{x \to 3^+} (6x + 2) = 6(3) + 2 = 18 + 2 = 20$.
3. Functional value: $f(3) = 6(3) + 2 = 20$.
Since the $\text{LHL} (-6) \neq \text{RHL} (20)$, the limit $\lim_{x \to 3} f(x)$ does not exist. Therefore, the function has a jump discontinuity at $x = 3$ [Per Definition of Point of Discontinuity].
Visual Representation of Discontinuity
Summary of Continuity Analysis
| Point | Condition | Result |
|---|---|---|
| $x = -3$ | $\text{LHL} = \text{RHL} = f(-3)$ | Continuous |
| $x = 3$ | $\text{LHL} \neq \text{RHL}$ | Discontinuous |
Final Solution: The function $f(x)$ is continuous everywhere except at $x = 3$, where it has a jump discontinuity.
More Questions from Class 12 Mathematics Continuity and Differentiability EXERCISE 5.1
- Q1: Prove that the function $f(x) = 5x - 3$ is continuous at $x = 0$, at $x = -3$ and at $x = 5$.
- Q2: Examine the continuity of the function $f(x) = 2x^2 - 1$ at $x = 3$.
- Q3(a): Examine the following functions for continuity. (a) $f(x) = x - 5$
- Q3(b): Examine the following functions for continuity. (b) $f(x) = \frac{1}{x-5}$, $x \neq 5$
- Q3(c): Examine the following functions for continuity. (c) $f(x) = \frac{x^2 - 25}{x+5}$, $x \neq -5$
- Q3(d): Examine the following functions for continuity. (d) $f(x) = |x - 5|$
- Q4: Prove that the function $f(x) = x^n$ is continuous at $x = n$, where $n$ is a positive integer.
- Q5: Is the function $f$ defined by $f(x) = \begin{cases} x, & \text{if } x \le 1 \\ 5, & \text{if } x > 1 \end{cases}$ continuous at $x = 0$? At $x = 1$? At $x = 2$?
- Q6: Find all points of discontinuity of $f$, where $f$ is defined by $f(x) = \begin{cases} 2x + 3, & \text{if } x \le 2 \\ 2x - 3, & \text{if } x > 2 \end{cases}$
- Q8: Find all points of discontinuity of $f$, where $f$ is defined by $f(x) = \begin{cases} \frac{|x|}{x}, & \text{if } x \neq 0 \\ 0, & \text{if } x = 0 \end{cases}$
- Q9: Find all points of discontinuity of $f$, where $f$ is defined by $f(x) = \begin{cases} \frac{x}{|x|}, & \text{if } x < 0 \\ -1, & \text{if } x \ge 0 \end{cases}$
- Q10: Find all points of discontinuity of $f$, where $f$ is defined by $f(x) = \begin{cases} x + 1, & \text{if } x \ge 1 \\ x^2 + 1, & \text{if } x < 1 \end{cases}$
- Q11: Find all points of discontinuity of $f$, where $f$ is defined by $f(x) = \begin{cases} x^3 - 3, & \text{if } x \le 2 \\ x^2 + 1, & \text{if } x > 2 \end{cases}$
- Q12: Find all points of discontinuity of $f$, where $f$ is defined by $f(x) = \begin{cases} x^{10} - 1, & \text{if } x \le 1 \\ x^2, & \text{if } x > 1 \end{cases}$
- Q13: Is the function defined by $f(x) = \begin{cases} x + 5, & \text{if } x \le 1 \\ x - 5, & \text{if } x > 1 \end{cases}$ a continuous function?
- Q14: Discuss the continuity of the function $f$, where $f$ is defined by $f(x) = \begin{cases} 3, & \text{if } 0 \le x \le 1 \\ 4, & \text{if } 1 < x < 3 \\ 5, & \text{if } 3 \le x \le 10 \end{cases}$
- Q15: Discuss the continuity of the function $f$, where $f$ is defined by $f(x) = \begin{cases} 2x, & \text{if } x < 0 \\ 0, & \text{if } 0 \le x \le 1 \\ 4x, & \text{if } x > 1 \end{cases}$
- Q16: Discuss the continuity of the function $f$, where $f$ is defined by $f(x) = \begin{cases} -2, & \text{if } x \le -1 \\ 2x, & \text{if } -1 < x \le 1 \\ 2, & \text{if } x > 1 \end{cases}$
- Q17: Find the relationship between $a$ and $b$ so that the function $f$ defined by $f(x) = \begin{cases} ax + 1, & \text{if } x \le 3 \\ bx + 3, & \text{if } x > 3 \end{cases}$ is continuous at $x = 3$.
- Q18: For what value of $\lambda$ is the function defined by $f(x) = \begin{cases} \lambda(x^2 - 2x), & \text{if } x \le 0 \\ 4x + 1, & \text{if } x > 0 \end{cases}$ continuous at $x = 0$? What about continuity at $x = 1$?
- Q19: Show that the function defined by $g(x) = x - [x]$ is discontinuous at all integral points. Here $[x]$ denotes the greatest integer less than or equal to $x$.
- Q20: Is the function defined by $f(x) = x^2 - \sin x + 5$ continuous at $x = \pi$?
- Q21(a): Discuss the continuity of the following functions: (a) $f(x) = \sin x + \cos x$
- Q21(b): Discuss the continuity of the following functions: (b) $f(x) = \sin x - \cos x$
- Q21(c): Discuss the continuity of the following functions: (c) $f(x) = \sin x \cdot \cos x$
- Q22: Discuss the continuity of the cosine, cosecant, secant and cotangent functions.
- Q23: Find all points of discontinuity of $f$, where $f(x) = \begin{cases} \frac{\sin x}{x}, & \text{if } x < 0 \\ x + 1, & \text{if } x \ge 0 \end{cases}$
- Q24: Determine if $f$ defined by $f(x) = \begin{cases} x^2 \sin \frac{1}{x}, & \text{if } x \neq 0 \\ 0, & \text{if } x = 0 \end{cases}$ is a continuous function?
- Q25: Examine the continuity of $f$, where $f$ is defined by $f(x) = \begin{cases} \sin x - \cos x, & \text{if } x \neq 0 \\ -1, & \text{if } x = 0 \end{cases}$
- Q26: Find the values of $k$ so that the function $f$ is continuous at the indicated point in Exercises 26 to 29. $f(x) = \begin{cases} \frac{k \cos x}{\pi - 2x}, & \text{if } x \neq \frac{\pi}{2} \\ 3, & \text{if } x = \frac{\pi}{2} \end{cases}$ at $x = \frac{\pi}{2}$
- Q27: Find the values of $k$ so that the function $f$ is continuous at the indicated point in Exercises 26 to 29. $f(x) = \begin{cases} kx^2, & \text{if } x \le 2 \\ 3, & \text{if } x > 2 \end{cases}$ at $x = 2$
- Q28: Find the values of $k$ so that the function $f$ is continuous at the indicated point in Exercises 26 to 29. $f(x) = \begin{cases} kx + 1, & \text{if } x \le \pi \\ \cos x, & \text{if } x > \pi \end{cases}$ at $x = \pi$
- Q29: Find the values of $k$ so that the function $f$ is continuous at the indicated point in Exercises 26 to 29. $f(x) = \begin{cases} kx + 1, & \text{if } x \le 5 \\ 3x - 5, & \text{if } x > 5 \end{cases}$ at $x = 5$
- Q30: Find the values of $a$ and $b$ such that the function defined by $f(x) = \begin{cases} 5, & \text{if } x \le 2 \\ ax + b, & \text{if } 2 < x < 10 \\ 21, & \text{if } x \ge 10 \end{cases}$ is a continuous function.
- Q31: Show that the function defined by $f(x) = \cos(x^2)$ is a continuous function.
- Q32: Show that the function defined by $f(x) = |\cos x|$ is a continuous function.
- Q33: Examine that $\sin|x|$ is a continuous function.
- Q34: Find all the points of discontinuity of $f$ defined by $f(x) = |x| - |x + 1|$.
CBSE Solutions for Class 12 Mathematics Continuity and Differentiability
- Continuity and Differentiability Class 12 EXERCISE 5.2
- Continuity and Differentiability Class 12 EXERCISE 5.3
- Continuity and Differentiability Class 12 EXERCISE 5.4
- Continuity and Differentiability Class 12 EXERCISE 5.5
- Continuity and Differentiability Class 12 EXERCISE 5.6
- Continuity and Differentiability Class 12 EXERCISE 5.7
Chapters in CBSE - Class 12 Mathematics
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