You can put this solution on YOUR website! A non-linear function is a function where, for some constant a, a*f(x) != f(a*x) for some x.
Some examples would be:
Trig functions: f(x) = sin (x) or f(x) = cos (x), etc.
Natural log: f(x) = ln(x)
Exponential: f(x) = e^x
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