\[\boxed{\mathbf{21}\mathbf{.}}\]
\[\textbf{а)}\ y = \arcsin{(\sin x)}\ \]
\[\textbf{б)}\ y = \arcsin{(\cos x)}\]
\[\textbf{в)}\ y = \arcsin{(tgx)}\]
\[\textbf{г)}\ y = \arcsin{(ctgx)}\]
\[\textbf{д)}\ y = \arccos{(\cos x)}\]
\[\textbf{е)}\ y = \arccos\left( \sin x \right)\]
\[\textbf{ж)}\ y = \arccos{(tgx)}\]
\[\textbf{з)}\ y = \arccos{(ctgx)}\]
\[\textbf{и)}\ y = arctg\left( \text{tgx} \right)\]
\[к)\ y = arctg(ctgx)\]
\[л)\ y = arctg\left( \sin x \right)\]
\[м)\ y = arctg(\cos x)\]
\[н)\ y = arcctg(ctg\ x)\]
\[о)\ y = arcctg(tgx)\]
\[п)\ y = arcctg(\sin x)\]
\[р)\ y = arcctg\left( \cos x \right)\]