Practicing Success

Target Exam

CUET

Subject

General Test

Chapter

Quantitative Reasoning

Topic

Trigonometry

Question:

If $\sin ^6 \theta+\cos ^6 \theta=\frac{1}{3}, 0^{\circ}<\theta<90^{\circ}$, then what is the value of $\sin \theta \cos \theta$ ?

Options:

$\frac{\sqrt{2}}{3}$

$\frac{\sqrt{6}}{6}$

$\frac{\sqrt{2}}{\sqrt{3}}$

$\frac{2}{9}$

Correct Answer:

$\frac{\sqrt{2}}{3}$

Explanation:

sin6 θ + cos6 θ = \(\frac{1}{3}\)

(sin² θ)³ + (cos² θ)³ = \(\frac{1}{3}\)

( sin² θ + cos² θ ) . ( sin4 θ + cos4 θ -  sinθ.cosθ ) = \(\frac{1}{3}\)

{ using , sin² θ + cos² θ = 1 }

( sin4 θ + cos4 θ -  sin²θ.cos²θ ) = \(\frac{1}{3}\)

( sin² θ + cos² θ ) -  sin²θ.cos²θ  = \(\frac{1}{3}\)

( sin² θ + cos² θ ) - 3 sin²θ.cos²θ  = \(\frac{1}{3}\)

1 - 3 sin²θ.cos²θ  = \(\frac{1}{3}\)

1 - \(\frac{1}{3}\) = 3 sin²θ.cos²θ

\(\frac{2}{9}\) =  sin²θ.cos²θ

sinθ.cosθ = \(\frac{√2}{3}\)