A Taylor series is a power series expansion of a function $f(x)$ about a given point. The Taylor series expansion of a differentiable function $f(x)$ about a point $x=c$ is given by:

The Taylor series expansion about the point $c=0$ is known as a Maclaurin series expansion.

Taylor series expansions have many applications, including evaluating definite integrals, finding the limit of a function and approximating the value of an expression.

Worked Example

Example 1

Use the first two terms of a Taylor expansion to approximate $\cos{\left(\dfrac{4\pi}{5}\right)}$.

Solution

Here the function to expand is $f(x)=\cos{x}$. Recall that the first two terms of a Taylor expansion about the point $x=c$ are given by \[f(x)\approx f(c)+f'(c)(x-c).\]

The derivative of $\cos{x}$ with respect to $x$ is $-\sin{x}$, so the first two terms of the Taylor series expansion for $\cos{x}$ are: \[f(x)\approx \cos{c}-(x-c)\sin{c}.\]

To approximate $\cos{\left(\dfrac{4\pi}{5}\right)}$, a suitable choice for $c$ must be made.

First note that: \[\frac{4\pi}{5}=\frac{16\pi}{20}=\frac{(15+1)\pi}{20}=\frac{3\pi}{4}+\frac{\pi}{20}\].

Since $\cos{\left(\dfrac{3\pi}{4}\right)}$ and $\sin{\left(\dfrac{3\pi}{4}\right)}$ are commonly known trigonometric ratios, choose $c=\dfrac{3\pi}{4}$. Substituting $x=\dfrac{4\pi}{5}$ and $c=\dfrac{3\pi}{4}$ into the expansion gives