# Power series representation/calculation

I am struggling a bit with power series at the moment, and I don’t quite understand what this question is asking me to do? Am I meant to form a power series from these, or simply evaluate that series? Any explanation/working is appreciated.

Using power series representation, calculate

$$\sum_{n=1}^\infty \frac{n2^n}{3^n}.$$

#### Solutions Collecting From Web of "Power series representation/calculation"

Hint: Define $f$ by

$$f(x) = \sum_{n = 1}^{\infty} x^n$$

Using the formula for a geometric series, this is the same as

$$f(x) = \frac{x}{1 – x}$$

Now compute $f’$ two different ways.

Recall that, in general,
$$1 + x + x^2 + \cdots = \frac{1}{1 – x}, \quad |x| < 1.$$

Moreover, power series can be differentiated term by term. So, differentiating both sides of the equation above we get
$$1 + 2x + 3x^2 + \cdots = \frac{1}{(1 – x)^2}, \quad |x| < 1.$$

Now, multiplying both sides by $x$ leads to
$$x + 2x^2 + 3x^3 + \cdots = \sum_{n = 1}^\infty nx^n = \frac{x}{(1 – x)^2}, \quad |x| < 1.$$

However, in this case $x = 2/3 < 1$, so simply substitute $x = 2/3$ in formula above.

$$\sum_{x = 0}^{\infty} x ^ n = \frac{1}{1 – x}.$$
And then you do all the mathematical operations such as $\frac{d}{dx}$ on both sides until you get the form you want. For example, the first derivative will give you
$$\sum_{x = 1}^{\infty} n x ^ {n-1} = \frac{1}{(1 – x) ^ 2}.$$
A popular second step you can do from there is multiply both sides by $x$, which gives you
$$\sum_{x = 1}^{\infty} n x ^ {n} = \frac{x}{(1 – x) ^ 2}.$$