`D = RR \\ {k(π)/2 | k in ZZ}`
`4tan x + cot x = 5`
`-> 4tan x + 1/(tan x) - 5 = 0`
`-> 4tan^2 x - 5tan x + 1 = 0`
`->` \(\left[ \begin{array}{l}tan x = 1\\tan x = \dfrac{1}{4}\end{array} \right.\)
`->` \(\left[ \begin{array}{l}x = \dfrac{π}{4} + kπ\\x = arctan \dfrac{1}{4} + kπ\end{array} \right.\) `(k in ZZ)`
`sin x + cos x = sqrt{2}cos 3x`
`-> 1/(\sqrt{2})sin x + 1/(\sqrt{2})cos x = cos 3x`
`-> cos (π/4 - x) = cos 3x`
`->` \(\left[ \begin{array}{l}\dfrac{π}{4} - x = 3x + k2π\\\dfrac{π}{4} - x = -3x + k2π\end{array} \right.\)
`->` \(\left[ \begin{array}{l}x = -\dfrac{π}{16} + k\dfrac{π}{2}\\x = -\dfrac{π}{8} + kπ\end{array} \right.\) `(k in ZZ)`
