\starttext

Using ^2:

\placeformula
\startformula
\left( a + b \right)^2 = a² + 2ab + b²
\stopformula

\placeformula
\startformula
\left( a + \frac72 \right)^2
\stopformula

Using ². The first example does not display the problem that well because the parenthesis are too small:
\placeformula
\startformula
\left( a + b \right)² = a² + 2ab + b²
\stopformula

\placeformula
\startformula
\left( n + \frac72 \right)²
\stopformula
\stoptext