Using the Func as mentioned above works but there are also delegates that do the same task and also define intent within the naming:
public delegate double MyFunction(double x);
public double Diff(double x, MyFunction f)
{
double h = 0.0000001;
return (f(x + h) - f(x)) / h;
}
public double MyFunctionMethod(double x)
{
// Can add more complicated logic here
return x + 10;
}
public void Client()
{
double result = Diff(1.234, x => x * 456.1234);
double secondResult = Diff(2.345, MyFunctionMethod);
}