H itself tends to zero.

So using Robida's law,

Limit = (derivative of numerator at h=0)/(derivative of denominator at h=0)

Is (a h)'/ 1 = (a h) lna。

When h tends to 0, a h = 1.

So the limit is lna

Multiply the previous (a to the power of x)

The result is (a x) lna.