The functions f and g are defined for \(-\frac{1}{2}\pi \leq x \leq \frac{1}{2}\pi\) by
\(f(x) = \frac{1}{2}x + \frac{1}{6}\pi\),
\(g(x) = \cos x\).
Solve the following equations for \(-\frac{1}{2}\pi \leq x \leq \frac{1}{2}\pi\).
- \(gf(x) = 1\), giving your answer in terms of \(\pi\).
- \(fg(x) = 1\), giving your answers correct to 2 decimal places.
Solution
(i) We have \(gf(x) = 1\), which means \(g(f(x)) = 1\). Therefore, \(\cos\left(\frac{1}{2}x + \frac{\pi}{6}\right) = 1\).
The cosine function equals 1 at \(0\), so \(\frac{1}{2}x + \frac{\pi}{6} = 0\).
Solving for \(x\), we get:
\(\frac{1}{2}x = -\frac{\pi}{6}\)
\(x = -\frac{\pi}{3}\).
(ii) We have \(fg(x) = 1\), which means \(f(g(x)) = 1\). Therefore, \(\frac{1}{2}\cos x + \frac{\pi}{6} = 1\).
Solving for \(\cos x\), we get:
\(\frac{1}{2}\cos x = 1 - \frac{\pi}{6}\)
\(\cos x = 2\left(1 - \frac{\pi}{6}\right)\)
\(\cos x = 0.9528\)
Solving for \(x\), we find \(x = \pm 0.31\) (to 2 decimal places).
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