If tan4 θ + tan2 θ = 1, what is the value of 11 (cos4 θ + cos2 θ)?
Show answer & explanation
Correct answer: D
Explanation
The correct answer is option 4 i.e. 11.
Tan4θ + Tan2θ = 1
⇒ Tan2θ (Tan2θ + 1) = 1
⇒ Tan2θ × Sec2θ = 1 {∵ sec2θ = 1 + tan2θ}
⇒ (sin2 θ/cos4 θ) = 1
⇒ 1 – cos2θ = cos4 θ {∵ sin2 θ = 1 – cos2θ}
⇒ cos4 θ + cos2 θ = 1
Hence
⇒ 11(cos4 θ + cos2 θ) = 1 × 11 = 11