To better understand: draw 2 associated right-angled triangles:
1) angle theta, with sides 2, root 5, 3
2) angle phi, with sides 3,4,5
cos \theta = \frac{2}{3} $ $ sin \theta = \frac {\sqrt 5}{3} \\ \\ tan \phi = \frac {3}{4} $ $ sin \phi = \frac {3}{5} $ $ cos \phi = \frac {4}{5} \\...