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.