In OEF the solution with array is OK if you replace by \lbrace
or \lpar or \lbracket according to the type of parenthesis
and of course \rbrace, etc for the right parenthesis if useful.
\statement{
\(\left\lbrace
\begin{array}{lclr}
f(x) & = & \sin(x^{2}) & (1) \\
h(x) & = & \sin\left(x^{\frac{1}{2}}\right) & (2) \\
i(x) & = & \sin(2x) & (3) \\
j(x) & = & \sin(3\cdot x) & (4) \\
k(x) & = & \sin(\sqrt{x}) & (5)
\end{array}
\right.
\)
}