- Prove the identity \[\cos \theta - 2 \cos 3 \theta + \cos 5\theta = 2\sin \theta (\sin 2 \theta - \sin 4 \theta).\]
Can we combine two \(\cos\) terms using an expression for the difference of two cosines?
We could use this twice here…
Solve the equations
\(\cos 2x = \sin x\),
\(3 \sec^2 x = \tan x + 5\)
giving in each case all solutions between \(0^\circ\) and \(360^\circ\).
Can we write the first equation just in terms of \(\sin x\)?
How can we relate \(\sec^2 x\) to \(\tan x\)?
This would reduce the equations to quadratic ones…