> For the complete documentation index, see [llms.txt](https://yingshaoxo.gitbook.io/university-notes/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://yingshaoxo.gitbook.io/university-notes/high-level-math/function-limitation-and-continuity-of-function/a-model-for-getting-limitation.md).

# A model for getting limitation

## Method

If anything goes well, it indeed has a general model for you guys getting limitation from a limit equation.

```
if an equation is a fraction:
    if fraction + or - a fraction:
        reduce fractions into one fraction by setting a common denominator
    if the equation has a radical sign:
        get rid of it by rationalizing.
    if put giving_x into equation getting "0/0":
        do factorization, get rid of one factor which makes equation looks like "0/0"
if the equation is not a fraction: # combined with simple +-x/
    put giving_x in equation directly
```

## Example

$$
\begin{align\*}
& \lim\_{x \to 1}{\frac{x^2 + 2x - 3}{x - 1}}
\ \\
\= & \lim\_{x \to 1}{\frac{(x + 3)(x - 1)}{x - 1}}
\ \\
\= & \lim\_{x \to 1}{\frac{x + 3}{1}}
\ \\
\= & 1 + 3
\ \\
\= & 4
\end{align\*}
$$

$$
\begin{align\*}
& \lim\_{x \to +\infty}{\sqrt{x^2 - 1} - x}
\ \\
\= & \lim\_{x \to +\infty}{\frac{\sqrt{x^2 - 1} - x}{1}}
\ \\
\= & \lim\_{x \to +\infty}{\frac{(\sqrt{x^2 - 1} - x) (\sqrt{x^2 - 1} + x)}{1(\sqrt{x^2 - 1} + x)}}
\ \\
\= & \lim\_{x \to +\infty}{\frac{x^2 - 1 - x^2}{\sqrt{x^2 - 1} + x}}
\ \\
\= & \lim\_{x \to +\infty}{\frac{-1}{\sqrt{x^2 - 1} + x}}
\ \\
\= & \lim\_{x \to +\infty}{\frac{-1}{\frac{\sqrt{x^2 - 1} + x}{x}}}
\ \\
\= & \lim\_{x \to +\infty}{\frac{-1}{\sqrt{\frac{x^2}{x^2} - \frac{1}{x^2}} + \frac{x}{x}}}
\ \\
\= & \lim\_{x \to +\infty}{\frac{-1}{\sqrt{1 - \frac{1}{x^2}} + 1}}
\ \\
\= & \frac{-1}{\sqrt{1 - \frac{1}{+\infty}} + 1}
\ \\
\= & \frac{-1}{\sqrt{1 - 0} + 1}
\ \\
\= & -\frac{1}{2}
\end{align\*}
$$