ID: Class: 11Subject: MathTopic: Trigonometric FunctionsType: Long
Question:
Find \(\sin\frac{x}{2}, \cos\frac{x}{2}, \tan\frac{x}{2}\) given \(\sin x = \frac{1}{4}\), and \(x\) lies in the second quadrant
Official Solution
Explanation:
\(\textbf{Step 1: Determine quadrant of \(\frac{x}{2}\)}\)
Since \(x\) lies in the second quadrant, i.e., \(\frac{\pi}{2} < x < \pi\), then:
\[
\frac{\pi}{4} < \frac{x}{2} < \frac{\pi}{2}
\]
So, \(\frac{x}{2}\) lies in the first quadrant. Therefore:
\[
\sin\left(\frac{x}{2}\right) > 0,\quad \cos\left(\frac{x}{2}\right) > 0,\quad \tan\left(\frac{x}{2}\right) > 0
\]
\(\textbf{Step 2: Use Pythagorean identity to find \(\cos x\)}\)
\[
\sin^2 x + \cos^2 x = 1 \Rightarrow \left(\frac{1}{4}\right)^2 + \cos^2 x = 1
\Rightarrow \cos^2 x = 1 - \frac{1}{16} = \frac{15}{16}
\Rightarrow \cos x = -\frac{\sqrt{15}}{4} \quad \text{(since \(x\) is in 2nd quadrant)}
\]
\(\textbf{Step 3: Use half-angle identity for sine}\)