# What is being Done in This proof of Limits?

1. Jul 11, 2013

2. Jul 11, 2013

### micromass

Staff Emeritus

If so, can you start by explaining what you think they're doing? And can you explain what you don't get??

3. Jul 11, 2013

This is another proof

4. Jul 11, 2013

### micromass

Staff Emeritus
It is very similar. So please, tell us what you think first.

5. Jul 11, 2013

In this part they arrive to the conclusion that Delta have to be < ε/10.

in the next step they arrive to the same conclusion. If δ <1 then δ<10. ( I may be wrong)

The next step is what become problematic for me to understand.

6. Jul 11, 2013

### Staff: Mentor

This problem is qualitatively different from the other one. In the earlier problem, the function was linear. Here the function is a quadratic.

In the second line, which is what I believe you're asking about, they make the assumption that $\delta < 1$. Then if $|x - 4 | < \delta < 1$, they can say that x will be between 3 and 5. Note that I'm ignoring the part where it says 0 < |x - 4|. All this does is eliminate the possibility of x being equal to 4.

Since 3 < x < 5, the largest that |x + 5| can be is 10. From this, they can write
$|x + 5||x - 4| < 10|x - 4|$
If we take $\delta = \epsilon/10$, then when $|x - 4| < \delta$, it will follow that
$|x + 5||x - 4| < 10|x - 4| < 10 * \delta < 10 * \epsilon/10 = \epsilon$