This MCQ module is based on: Properties of Inverse Trigonometric Functions
Properties of Inverse Trigonometric Functions
This mathematics assessment will be based on: Properties of Inverse Trigonometric Functions
Targeting Class 12 level in Trigonometry, with Advanced difficulty.
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2.3 Properties of Inverse Trigonometric Functions
In this section, we establish some important properties of inverse trigonometric functions. These results are valid within the principal value branches? of the corresponding functions and wherever they are defined.
Property Set 1: Cancellation Properties
- \(\sin(\sin^{-1} x) = x\), for \(x \in [-1, 1]\) and \(\sin^{-1}(\sin x) = x\), for \(x \in \left[-\frac{\pi}{2}, \frac{\pi}{2}\right]\)
- \(\cos(\cos^{-1} x) = x\), for \(x \in [-1, 1]\) and \(\cos^{-1}(\cos x) = x\), for \(x \in [0, \pi]\)
- \(\tan(\tan^{-1} x) = x\), for \(x \in \mathbf{R}\) and \(\tan^{-1}(\tan x) = x\), for \(x \in \left(-\frac{\pi}{2}, \frac{\pi}{2}\right)\)
Property Set 2: Negative Argument Identities
- \(\sin^{-1}(-x) = -\sin^{-1}(x)\), for \(x \in [-1, 1]\) (odd function)
- \(\tan^{-1}(-x) = -\tan^{-1}(x)\), for \(x \in \mathbf{R}\) (odd function)
- \(\csc^{-1}(-x) = -\csc^{-1}(x)\), for \(|x| \geq 1\) (odd function)
- \(\cos^{-1}(-x) = \pi - \cos^{-1}(x)\), for \(x \in [-1, 1]\)
- \(\sec^{-1}(-x) = \pi - \sec^{-1}(x)\), for \(|x| \geq 1\)
- \(\cot^{-1}(-x) = \pi - \cot^{-1}(x)\), for \(x \in \mathbf{R}\)
Even-like functions (cos, sec, cot): Their inverses satisfy \(f^{-1}(-x) = \pi - f^{-1}(x)\) (supplement rule).
Property Set 3: Complementary Pair Identities
- \(\sin^{-1}(x) + \cos^{-1}(x) = \frac{\pi}{2}\), for \(x \in [-1, 1]\)
- \(\tan^{-1}(x) + \cot^{-1}(x) = \frac{\pi}{2}\), for \(x \in \mathbf{R}\)
- \(\csc^{-1}(x) + \sec^{-1}(x) = \frac{\pi}{2}\), for \(|x| \geq 1\)
Property Set 4: Reciprocal Identities
- \(\sin^{-1}\left(\frac{1}{x}\right) = \csc^{-1}(x)\), for \(|x| \geq 1\)
- \(\cos^{-1}\left(\frac{1}{x}\right) = \sec^{-1}(x)\), for \(|x| \geq 1\)
- \(\tan^{-1}\left(\frac{1}{x}\right) = \cot^{-1}(x)\), for \(x > 0\)
Property Set 5: Addition Formulas for tan¹
Subtraction form: \[\tan^{-1} x - \tan^{-1} y = \tan^{-1}\left(\frac{x - y}{1 + xy}\right), \quad xy > -1\]
Property Set 6: Double Angle Formulas
- \(2\sin^{-1} x = \sin^{-1}(2x\sqrt{1-x^2})\), for \(-\frac{1}{\sqrt{2}} \leq x \leq \frac{1}{\sqrt{2}}\)
- \(2\cos^{-1} x = \cos^{-1}(2x^2 - 1)\), for \(0 \leq x \leq 1\)
- \(2\tan^{-1} x = \tan^{-1}\left(\frac{2x}{1-x^2}\right)\), for \(-1 < x < 1\)
- \(2\tan^{-1} x = \sin^{-1}\left(\frac{2x}{1+x^2}\right)\), for \(-1 \leq x \leq 1\)
- \(2\tan^{-1} x = \cos^{-1}\left(\frac{1-x^2}{1+x^2}\right)\), for \(x \geq 0\)
Worked Example 3
Show that:
(i) \(\sin^{-1}\left(2x\sqrt{1-x^2}\right) = 2\sin^{-1} x\), for \(-\frac{1}{\sqrt{2}} \leq x \leq \frac{1}{\sqrt{2}}\)
(ii) \(\sin^{-1}\left(2x\sqrt{1-x^2}\right) = 2\cos^{-1} x\), for \(\frac{1}{\sqrt{2}} \leq x \leq 1\)
\(2x\sqrt{1 - x^2} = 2\sin\theta\sqrt{1 - \sin^2\theta} = 2\sin\theta\cos\theta = \sin 2\theta\)
So \(\sin^{-1}(2x\sqrt{1-x^2}) = \sin^{-1}(\sin 2\theta) = 2\theta = 2\sin^{-1} x\).
(Valid when \(-\frac{\pi}{4} \leq \theta \leq \frac{\pi}{4}\), i.e., \(-\frac{1}{\sqrt{2}} \leq x \leq \frac{1}{\sqrt{2}}\).)
(ii) Let \(x = \cos \theta\). Then proceeding similarly, we get \(2\cos^{-1} x\).
Worked Example 4
Express \(\tan^{-1}\left(\frac{\cos x}{1 - \sin x}\right)\), \(-\frac{3\pi}{2} < x < \frac{\pi}{2}\), in the simplest form.
\[\frac{\cos x}{1 - \sin x} = \frac{\cos^2\frac{x}{2} - \sin^2\frac{x}{2}}{\left(\cos\frac{x}{2} - \sin\frac{x}{2}\right)^2} = \frac{\left(\cos\frac{x}{2} + \sin\frac{x}{2}\right)\left(\cos\frac{x}{2} - \sin\frac{x}{2}\right)}{\left(\cos\frac{x}{2} - \sin\frac{x}{2}\right)^2}\] \[= \frac{\cos\frac{x}{2} + \sin\frac{x}{2}}{\cos\frac{x}{2} - \sin\frac{x}{2}} = \frac{1 + \tan\frac{x}{2}}{1 - \tan\frac{x}{2}} = \tan\left(\frac{\pi}{4} + \frac{x}{2}\right)\] Therefore, \(\tan^{-1}\left(\frac{\cos x}{1 - \sin x}\right) = \frac{\pi}{4} + \frac{x}{2}\).
Worked Example 5
Write \(\cot^{-1}\left(\frac{1}{\sqrt{x^2 - 1}}\right)\), \(x > 1\) in the simplest form.
\(\cot^{-1}\left(\frac{1}{\sqrt{x^2-1}}\right) = \cot^{-1}(\cot \theta) = \theta = \sec^{-1} x\), which is the simplest form.
Exercise 2.2
Prove the following:
RHS = \(\sin^{-1}(3\sin\theta - 4\sin^3\theta) = \sin^{-1}(\sin 3\theta) = 3\theta = 3\sin^{-1} x\) = LHS.
(Since \(3\theta \in \left[-\frac{\pi}{2}, \frac{\pi}{2}\right]\), the principal value branch applies.)
RHS = \(\cos^{-1}(4\cos^3\theta - 3\cos\theta) = \cos^{-1}(\cos 3\theta) = 3\theta = 3\cos^{-1} x\) = LHS.
(Since \(3\theta \in [0, \pi]\), the principal value branch applies.)
Write the following functions in the simplest form:
\(\frac{\sec\theta - 1}{\tan\theta} = \frac{1 - \cos\theta}{\sin\theta} = \frac{2\sin^2\frac{\theta}{2}}{2\sin\frac{\theta}{2}\cos\frac{\theta}{2}} = \tan\frac{\theta}{2}\)
So the expression = \(\tan^{-1}\left(\tan\frac{\theta}{2}\right) = \frac{\theta}{2} = \frac{1}{2}\tan^{-1} x\).
So \(\sqrt{\frac{1-\cos x}{1+\cos x}} = \left|\tan\frac{x}{2}\right| = \tan\frac{x}{2}\) (since \(0 < x < \pi\) means \(0 < \frac{x}{2} < \frac{\pi}{2}\)).
Therefore, \(\tan^{-1}\left(\tan\frac{x}{2}\right) = \frac{x}{2}\).
\(\frac{1 - \tan x}{1 + \tan x} = \tan\left(\frac{\pi}{4} - x\right)\)
So \(\tan^{-1}\left(\frac{\cos x - \sin x}{\cos x + \sin x}\right) = \frac{\pi}{4} - x\).
Find the values of each of the following:
\(\tan\frac{3\pi}{4} = \tan\left(\pi - \frac{\pi}{4}\right) = -\tan\frac{\pi}{4} = -1\).
So \(\tan^{-1}\left(\tan\frac{3\pi}{4}\right) = \tan^{-1}(-1) = -\frac{\pi}{4}\).
\(\sin\frac{2\pi}{3} = \sin\left(\pi - \frac{\pi}{3}\right) = \sin\frac{\pi}{3} = \frac{\sqrt{3}}{2}\).
\(\sin^{-1}\left(\frac{\sqrt{3}}{2}\right) = \frac{\pi}{3}\) (which lies in \(\left[-\frac{\pi}{2}, \frac{\pi}{2}\right]\)).
So \(\cos^{-1} x = \frac{\pi}{2} - \sin^{-1}\frac{1}{5} = \cos^{-1}\frac{1}{5}\).
(Using: \(\sin^{-1} a + \cos^{-1} a = \frac{\pi}{2}\), so \(\frac{\pi}{2} - \sin^{-1}\frac{1}{5} = \cos^{-1}\frac{1}{5}\).)
Therefore \(x = \frac{1}{5}\).
Let \(a = \frac{x-1}{x-2}\), \(b = \frac{x+1}{x+2}\).
\(a + b = \frac{(x-1)(x+2) + (x+1)(x-2)}{(x-2)(x+2)} = \frac{x^2+x-2+x^2-x-2}{x^2-4} = \frac{2x^2-4}{x^2-4} = \frac{2(x^2-2)}{x^2-4}\)
\(1 - ab = 1 - \frac{(x-1)(x+1)}{(x-2)(x+2)} = 1 - \frac{x^2-1}{x^2-4} = \frac{x^2-4-x^2+1}{x^2-4} = \frac{-3}{x^2-4}\)
So \(\tan^{-1}\left(\frac{2(x^2-2)}{-3}\right) = \frac{\pi}{4}\).
\(\frac{2(x^2-2)}{-3} = \tan\frac{\pi}{4} = 1\), giving \(2(x^2-2) = -3\), so \(2x^2-4 = -3\), \(2x^2 = 1\), \(x^2 = \frac{1}{2}\).
\(x = \pm\frac{1}{\sqrt{2}}\).
- Choose \(x = 0\): \(\sin^{-1} 0 = 0\), \(\cos^{-1} 0 = \frac{\pi}{2}\). Sum = \(\frac{\pi}{2}\). Verified!
- Choose \(x = 1\): \(\sin^{-1} 1 = \frac{\pi}{2}\), \(\cos^{-1} 1 = 0\). Sum = \(\frac{\pi}{2}\). Verified!
- Choose \(x = -\frac{1}{2}\): \(\sin^{-1}\left(-\frac{1}{2}\right) = -\frac{\pi}{6}\), \(\cos^{-1}\left(-\frac{1}{2}\right) = \frac{2\pi}{3}\). Sum = \(-\frac{\pi}{6} + \frac{2\pi}{3} = \frac{-\pi + 4\pi}{6} = \frac{3\pi}{6} = \frac{\pi}{2}\). Verified!
- Try \(x = 2\): \(\sin^{-1}(2)\) is undefined. The identity only holds for \(x \in [-1, 1]\).
Observation: The identity holds for every value of \(x\) in the common domain \([-1, 1]\), but is meaningless outside it.
Proof sketch: Let \(\sin^{-1} x = \theta\). Then \(\theta \in \left[-\frac{\pi}{2}, \frac{\pi}{2}\right]\) and \(\sin \theta = x\). Now \(\cos\left(\frac{\pi}{2} - \theta\right) = \sin \theta = x\). Since \(\frac{\pi}{2} - \theta \in [0, \pi]\) (the range of \(\cos^{-1}\)), we get \(\cos^{-1} x = \frac{\pi}{2} - \theta = \frac{\pi}{2} - \sin^{-1} x\). Rearranging: \(\sin^{-1} x + \cos^{-1} x = \frac{\pi}{2}\).
Competency-Based Questions
\(\frac{R}{40} = \sin 60^\circ = \frac{\sqrt{3}}{2}\), giving \(R = 20\sqrt{3} \approx 34.64\) m.
At this point, \(\alpha = \frac{1}{2} \times \frac{\pi}{2} = \frac{\pi}{4} = 45^\circ\). This matches the well-known result that a \(45^\circ\) launch angle maximizes range.
\(\alpha = \frac{1}{2}\left(\frac{\pi}{2} - \cos^{-1}\left(\frac{gR}{v^2}\right)\right) = \frac{\pi}{4} - \frac{1}{2}\cos^{-1}\left(\frac{gR}{v^2}\right)\).
Assertion–Reason Questions
Reason (R): \(\sin^{-1}(-x) = -\sin^{-1}(x)\) for all \(x \in [-1, 1]\).
Reason (R): \(\cos^{-1}(-x) = \pi - \cos^{-1}(x)\) for \(x \in [-1, 1]\).
Reason (R): \(\tan^{-1} x + \tan^{-1} y = \pi + \tan^{-1}\left(\frac{x+y}{1-xy}\right)\) when \(xy > 1\) and \(x, y > 0\).
Then \(\tan^{-1}(1) + \frac{3\pi}{4} = \frac{\pi}{4} + \frac{3\pi}{4} = \pi\). Both A and R are true, and R is used to prove A.
Frequently Asked Questions
What is sin-inverse(x) + cos-inverse(x)?
For all x in [-1, 1], sin-inverse(x) + cos-inverse(x) = pi/2. Similarly, tan-inverse(x) + cot-inverse(x) = pi/2 for all real x.
What are the key properties of inverse trig functions?
Key properties: sin-inverse(-x) = -sin-inverse(x), cos-inverse(-x) = pi - cos-inverse(x), tan-inverse addition formula, and double-angle conversions.
How to simplify expressions involving inverse trig functions?
Convert to a common inverse trig form, use complementary relations, apply double-angle formulas like 2tan-inverse(x) = sin-inverse(2x/(1+x^2)), and use addition formulas.
What is the formula for tan-inverse(x) + tan-inverse(y)?
tan-inverse(x) + tan-inverse(y) = tan-inverse((x+y)/(1-xy)) when xy < 1. Additional cases apply when xy > 1.
How to prove inverse trigonometric identities?
Let the expression equal theta, use the direct trig value, apply known identities, convert all terms to one type using standard conversions, and apply addition formulas.
Frequently Asked Questions — Inverse Trigonometric Functions
What is Properties of Inverse Trigonometric Functions in NCERT Class 12 Mathematics?
Properties of Inverse Trigonometric Functions is a key concept covered in NCERT Class 12 Mathematics, Chapter 2: Inverse Trigonometric Functions. This lesson builds the student's foundation in the chapter by explaining the core ideas with worked examples, definitions, and step-by-step methods aligned to the CBSE curriculum.
How do I solve problems on Properties of Inverse Trigonometric Functions step by step?
To solve problems on Properties of Inverse Trigonometric Functions, follow the NCERT method: identify the given quantities, choose the relevant formula or theorem, substitute values carefully, and simplify. Class 12 exercises gradually increase in difficulty — start with solved NCERT examples before attempting exercise questions, and always verify your answer by substitution or diagram.
What are the most important formulas for Chapter 2: Inverse Trigonometric Functions?
The essential formulas of Chapter 2 (Inverse Trigonometric Functions) are listed in the chapter summary and highlighted throughout the lesson in formula boxes. Memorise them and practise at least 2–3 problems per formula. CBSE board exams frequently test direct application as well as combined use of multiple formulas from this chapter.
Is Properties of Inverse Trigonometric Functions important for the Class 12 board exam?
Properties of Inverse Trigonometric Functions is part of the NCERT Class 12 Mathematics syllabus and appears in CBSE board exams. Questions typically include short-answer, long-answer, and competency-based items. Review the NCERT examples, exercise questions, and previous-year board problems on this topic to prepare confidently.
What mistakes should students avoid in Properties of Inverse Trigonometric Functions?
Common mistakes in Properties of Inverse Trigonometric Functions include skipping steps, misapplying formulas, sign errors, and losing track of units. Write each step clearly, double-check algebraic manipulations, and re-read the question after solving to verify that your answer matches what was asked.
Where can I find more NCERT practice questions on Properties of Inverse Trigonometric Functions?
End-of-chapter NCERT exercises for Properties of Inverse Trigonometric Functions cover all difficulty levels tested in CBSE exams. After completing them, try the examples again without looking at the solutions, attempt the NCERT Exemplar questions for Chapter 2, and solve at least one previous-year board paper to consolidate your understanding.