0606 P12 - Nov 2022 - Q4 - 5 marks
7863
Solve the equation
\(3\sin\left(2x+\frac{\pi}{4}\right)=\sqrt3\cos\left(2x+\frac{\pi}{4}\right),\)
for \(0\leq x\leq\pi\).
Solution
Answer: \(\displaystyle x=\frac{11\pi}{24}\) or \(\displaystyle x=\frac{23\pi}{24}\).
Answer: \(\displaystyle x=\frac{11\pi}{24}\) or \(\displaystyle x=\frac{23\pi}{24}\).
Let
\(u=2x+\frac{\pi}{4}.\)
The equation becomes
\(3\sin u=\sqrt3\cos u.\)
Since \(\cos u\neq0\) for the solutions of this equation, divide by \(\cos u\):
\(3\tan u=\sqrt3.\)
Therefore
\(\tan u=\frac{1}{\sqrt3}.\)
As \(0\leq x\leq\pi\),
\(\frac{\pi}{4}\leq u\leq\frac{9\pi}{4}.\)
In this interval,
\(u=\frac{7\pi}{6}\quad\text{or}\quad u=\frac{13\pi}{6}.\)
Hence
\(2x+\frac{\pi}{4}=\frac{7\pi}{6}\)
or
\(2x+\frac{\pi}{4}=\frac{13\pi}{6}.\)
Solving gives
\(x=\frac{11\pi}{24}\quad\text{or}\quad x=\frac{23\pi}{24}.\)