Practicing Success
If $\frac{sin\theta + cos \theta}{sin\theta - cos \theta}=3,$ then the value of $sin^4 \theta - cos^4 \theta $ is equal to : |
$\frac{1}{5}$ $\frac{2}{5}$ $\frac{3}{5}$ $\frac{4}{5}$ |
$\frac{3}{5}$ |
\(\frac{sin θ + cos θ}{sin θ - cos θ }\) = 3 sin θ + cos θ = 3sin θ - 3cos θ 2sin θ = 4cos θ tan θ = 2 Now, sin4 θ - cos4 θ = (sin² θ + cos² θ) . ( sin² θ - cos² θ ) = sin² θ - cos² θ { sin² θ + cos² θ= 1 } = - ( cos² θ - sin² θ ) = - cos2θ { cos² θ - sin² θ = cos2θ } = - \(\frac{1- tan² θ}{1+ tan² θ }\) = - \(\frac{1- 2²}{1+ 2² }\) = \(\frac{3}{5}\) |