Which of the following is/are identity/identities?

1. \(\frac{\sin ^3 \theta+\cos ^3 \theta}{\sin \theta+\cos \theta}+\sin \theta \cos \theta\) = \(1 ; 0<\theta<\frac{\pi}{2}\)

2. 1 - sin6 θ = cos2 θ (cos4 θ + 3 sinθ)

Select the correct answer using the code given below:

This question was previously asked in
CDS Elementary Mathematics 3 Sep 2023 Official Paper
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  1. 1 only
  2. 2 only
  3. Both 1 and 2
  4. Neither 1 nor 2

Answer (Detailed Solution Below)

Option 3 : Both 1 and 2
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Detailed Solution

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Formula used:

a3 + b3 = (a + b)(a2 + b2 - ab)

a3 - b3 = (a - b)(a2 + b2 + ab)

Calculation:

Statement: 1 \(\frac{\sin ^3 \theta+\cos ^3 \theta}{\sin \theta+\cos \theta}+\sin \theta \cos \theta\) = 1

Put θ = 45° [0< θ < π/2]

Sin 45° = 1/√2, Cos 45° = 1/√2

\(\frac{1}{2√2}\ +\ \frac{1}{2√2}\over\frac{1}{√2}\ +\ \frac{1}{√2}\) +\(\frac{1}{√2} \times \frac{1}{√2}\) = \(\frac{2}{2√2}\over\frac{2}{√2}\) + 1/2 = 1/2 + 1/2 = 1

Hence, Statement  1 is correct 

Statement: 2 1 - Sin6 θ = Cos2 θ(Cos4 θ + 3 Sinθ)

LHS: 13 - (Sin2 θ)3

 (1 -  Sin2 θ)[12 + (Sin2 θ)2 + Sin2 θ]

⇒ (1 -  Sin2 θ)[12 + Sin4 θSin2 θ]

RH:  (1 - Sin2 θ)[(1 - Sin2 θ)2 + 3 Sinθ]

⇒ (1 - Sin2 θ)[12 - 2Sin2 θ + (Sin2 θ)2 + 3 Sinθ]

⇒ (1 - Sin2 θ)[12 - 2Sin2 θ + Sin4 θ + 3 Sinθ]

⇒ (1 - Sin2 θ)[12 + Sin4 θ + Sinθ]

Hence statement 2 is correct

∴ Both the statement is correct

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