\placeformula[First]
\startformula
  \sin^2(t)^2 + \cos^2(t) = 1.
\stopformula

This is another formula

\placeformula[eqn:tangent]
\startformula
  \cos^2(t) = \frac{1}{1+\tan^2(t)}.
\stopformula

As one may see from relation~(\in[eqn:tangent]), $\cos(t)$ and $\sin(t)$ can be determined by $\tan(t)$\dots

This is the third formula

\placeformula[Pythagora]
\startformula
   |a+b|^2 = |a|^2 + |b|^2
\stopformula

Clearly equality (\in[Pythagora]) holds for $a,b \in {\Bbb R}^n$ if and only if $(a|b) =0$, where the scalar product $(\cdot|\cdot)$ on ${\Bbb R}^n$ is defined to be

\startformula
  (a|b) := a\cdot b =\sum_{j=1}^{n}a_jb_j, \qquad |a|^2 := (a|a).
\stopformula