Express \(5 y ^ { 2 } - 30 y + 50\) in the form \(5 ( y + a ) ^ { 2 } + b\), where \(a\) and \(b\) are constants.
The function f is defined by \(\mathrm { f } ( x ) = x ^ { 5 } - 10 x ^ { 3 } + 50 x\) for \(x \in \mathbb { R }\).
Determine whether \(f\) is an increasing function, a decreasing function or neither.