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Q3(i):
Choose the correct option. Justify your choice. (i) $9 \sec^2 A – 9 \tan^2 A =$
Choose the correct option. Justify your choice. (i) $9 \sec^2 A – 9 \tan^2 A =$
Solution :
Given: An algebraic expression involving trigonometric functions: $9 \sec^2 A - 9 \tan^2 A$.
To Find: The numerical value of the expression by simplifying it using trigonometric identities.
Step 1: Factoring the expression
The given expression is $9 \sec^2 A - 9 \tan^2 A$. We observe that the constant $9$ is common to both terms. We can factor it out using the distributive property of multiplication over subtraction:
$9 \sec^2 A - 9 \tan^2 A = 9(\sec^2 A - \tan^2 A)$
Step 2: Identifying the relevant trigonometric identity
We recall the fundamental trigonometric identity relating the secant and tangent functions:
$1 + \tan^2 A = \sec^2 A$
[Since the identity holds for all values of $A$ where the functions are defined]
Step 3: Rearranging the identity
To match the expression inside the parentheses, we rearrange the identity $1 + \tan^2 A = \sec^2 A$ by subtracting $\tan^2 A$ from both sides:
$1 = \sec^2 A - \tan^2 A$
[By the subtraction property of equality]
Step 4: Substitution and Final Calculation
Now, substitute the value of $(\sec^2 A - \tan^2 A)$ into the factored expression from Step 1:
$9(\sec^2 A - \tan^2 A) = 9(1)$
[Substituting $1$ for the expression $\sec^2 A - \tan^2 A$]
$9(1) = 9$
Justification: The expression simplifies to $9$ because the difference between the square of the secant and the square of the tangent of an angle is always equal to $1$, as derived from the Pythagorean identity $1 + \tan^2 A = \sec^2 A$.
Final Answer: 9
More Questions from Class 10 Mathematics Introduction to Trigonometry EXERCISE 8.3
- Q1: Express the trigonometric ratios $\sin A$, $\sec A$ and $\tan A$ in terms of $\cot A$.
- Q2: Write all the other trigonometric ratios of $\angle A$ in terms of $\sec A$.
- Q3(ii): Choose the correct option. Justify your choice. (ii) $(1 + \tan \theta + \sec \theta) (1 + \cot \theta – \text{cosec } \theta) =$
- Q3(iii): Choose the correct option. Justify your choice. (iii) $(\sec A + \tan A) (1 – \sin A) =$
- Q3(iv): Choose the correct option. Justify your choice. (iv) $\frac{1 + \tan^2 A}{1 + \cot^2 A} =$
- Q4(i): Prove the following identities, where the angles involved are acute angles for which the expressions are defined. (i) $(\text{cosec } \theta โ \cot \theta)^2 = \frac{1 - \cos \theta}{1 + \cos \theta}$
- Q4(ii): Prove the following identities, where the angles involved are acute angles for which the expressions are defined. (ii) $\frac{\cos A}{1 + \sin A} + \frac{1 + \sin A}{\cos A} = 2 \sec A$
- Q4(iii): Prove the following identities, where the angles involved are acute angles for which the expressions are defined. (iii) $\frac{\tan \theta}{1 - \cot \theta} + \frac{\cot \theta}{1 - \tan \theta} = 1 + \sec \theta \text{cosec } \theta$ [Hint : Write the expression in terms of $\sin \theta$ and $\cos \theta$]
- Q4(iv): Prove the following identities, where the angles involved are acute angles for which the expressions are defined. (iv) $\frac{1 + \sec A}{\sec A} = \frac{\sin^2 A}{1 โ \cos A}$ [Hint : Simplify LHS and RHS separately]
- Q4(ix): Prove the following identities, where the angles involved are acute angles for which the expressions are defined. (ix) $(\text{cosec } A โ \sin A) (\sec A โ \cos A) = \frac{1}{\tan A + \cot A}$ [Hint : Simplify LHS and RHS separately]
- Q4(v): Prove the following identities, where the angles involved are acute angles for which the expressions are defined. (v) $\frac{\cos A โ \sin A + 1}{\cos A + \sin A โ 1} = \text{cosec } A + \cot A$, using the identity $\text{cosec}^2 A = 1 + \cot^2 A$.
- Q4(vi): Prove the following identities, where the angles involved are acute angles for which the expressions are defined. (vi) $\sqrt{\frac{1 + \sin A}{1 โ \sin A}} = \sec A + \tan A$
- Q4(vii): Prove the following identities, where the angles involved are acute angles for which the expressions are defined. (vii) $\frac{\sin \theta - 2 \sin^3 \theta}{2 \cos^3 \theta - \cos \theta} = \tan \theta$
- Q4(viii): Prove the following identities, where the angles involved are acute angles for which the expressions are defined. (viii) $(\sin A + \text{cosec } A)^2 + (\cos A + \sec A)^2 = 7 + \tan^2 A + \cot^2 A$
- Q4(x): Prove the following identities, where the angles involved are acute angles for which the expressions are defined. (x) $(\frac{1 + \tan^2 A}{1 + \cot^2 A}) = (\frac{1 - \tan A}{1 - \cot A})^2 = \tan^2 A$
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