Video Transcript
Determine whether the series the
sum from 𝑛 equals one to ∞ of one over the square root of 𝑛 cubed converges
or diverges.
If we start by rewriting this sum,
using the fact that we can write the square root of 𝑎 as 𝑎 to the half power, we
can say that this sum is equivalent to the sum from 𝑛 equals one to ∞ of one
over 𝑛 cubed raised to the half power. We can then use the fact that 𝑎 to
the 𝑥 power, then raised to the 𝑦 power, equals 𝑎 to the 𝑥 multiplied by 𝑦
power. So we can write our sum as the sum
from 𝑛 equals one to ∞ of one over 𝑛 to the three over two power. And then we can recognise this to
be a 𝑝-series, which is a series of the form the sum from 𝑛 equals one to ∞
of one over 𝑛 to the 𝑝 power.
So let’s write out the conditions
for convergence for a 𝑝-series. The 𝑝-series the sum from 𝑛
equals one to ∞ of one over 𝑛 to the 𝑝 power is convergent if 𝑝 is greater
than one and divergent if 𝑝 is less than or equal to one. So for this series 𝑝 is equal to
three over two; this is the same as 1.5, which is greater than one. So this series converges.
