Approximate integral in one line of code
Version 1.
It gives incorrect results for some delta values.
Example: Integrate(x =&gt; x * x, 0, 1, 0.1) gives 0.285 whenever other methods give 0.385.

Version 2.
Since the delta is a scalar that is a constant value for the Enumerable.Sum, it could be moved outside of the Enumerable.Sum.
Then we can replace the c =&gt; f(c) * delta with a method group f.
And also we can use Enumerable's extension methods as extension methods:

static double Integrate(Func&lt;double, double&gt; f, double low, double high, double delta)
{
    return delta * DoubleRange(low, high, delta).Sum(f);
}

public static IEnumerable&lt;double&gt; DoubleRange(double min, double max, double step)
{
    for (double value = min; value &lt;= max; value += step)
    {
        yield return value;
    }
}


EDIT
The .Select(f).Sum() was replaced with the .Sum(f) as @mjolka advised.