what's more to Taylor series? Taylor series can be used to approximate any function, at a point, as a polynomial. And it's helpful because polynomial functions are really easy to differentiate. But, is there something more to it, or is it just a pleasing differentiation trick?
thank you :)
 A: The use of Taylor series is wide.
A cool example is the following (from complex analysis)
You might know from calc that the taylor expantion of $e^{x}$ is given by
$$e^x=\sum_{n=0}^\infty \frac{x^n}{n!}.$$
The thing is that, in the context of complex analysis, one can define $\exp$ in the entire complex plane. We define
$$\begin{align} \exp\colon \Bbb{C}& \to \Bbb{C}\\ z & \mapsto e^z=\sum_{n=0}^\infty\frac{z^n}{n!} \end{align}.$$
This is the most standard way of defining this function: it guaranties that this function is equal to the "original" exponential function whenever it is evaluated at a real value thus it is an extension of the exponential learned in calc. This leads to the following well known result (which has Taylor series as a fundamental argument):
$$\begin{align}e^{ix} & = \sum_{n=0}^\infty\frac{(ix)^n}{n!}\\
 &= \sum_{n=0}^\infty\frac{i^nx^n}{n!}\\
& = \sum_{n\text{ even}}^\infty\frac{i^nx^n}{n!}+\sum_{n\text{ odd}}^\infty\frac{i^nx^n}{n!}\\
& =\sum_{n=0}^\infty\frac{i^{2n}x^{2n}}{(2n)!}+\sum_{n=1}^\infty\frac{i^{2n-1}x^{2n-1}}{(2n-1)!} \\
 &= \sum_{n=0}^\infty\frac{(-1)^nx^{2n}}{(2n)!}+\sum_{n=1}^\infty\frac{i(-1)^{n+1}x^{2n-1}}{(2n-1)!}\\
 & = \sum_{n=0}^\infty\frac{(-1)^nx^{2n}}{(2n)!}+i\sum_{n=1}^\infty\frac{(-1)^{n+1}x^{2n-1}}{(2n-1)!}\\
 & = \cos x +i\sin x.
\end{align}$$
This result is known as Eulers identity. We can get an interesting result if we use this with $x=\pi$:
$$e^{i\pi}=-1$$
This result connects two extremely important, seemingly unrelated constants in mathematics, a result that follows by simple use of Taylor series.
You can also have applications (less theoretical, in contrast to the previous one); one can have numerical values for functions that can be calculated with computers.
This are just some example for the importance of Taylor series.
A: Taylor series can be very useful for limits and asymptotics, because you can truncate them while controlling the error in terms of higher powers of the distance to the expansion point. This is a very common application whenever we know that some argument is "small", e.g. because we're considering a discretisation.
As a practical example, famously the real-valued limit
$$
\lim_{x\to 0} \frac{\sin(x)}{x} 
$$
can be evaluated using l'Hôpital's rule. That, however, requires lots of assumption  checking. The Taylor expansion can instead immediately tell us that
$$
\sin(x) = 0 + x + \mathcal{O}(x^2)
$$
yielding
$$
\lim_{x\to 0} \frac{\sin(x)}{x} = \lim_{x\to 0} \frac{x + \mathcal{O}(x^2)} {x} = \lim_{x\to 0} \bigg(\frac{x}{x} + \mathcal{O}(x)\bigg) = 1
$$
If you want to practice this concept, try applying it to
$$
\lim_{x\to 0} \frac{1-\cos(x)}{x^2}
$$
