# How do I find the value of n?

## Homework Statement How do I find the value of n?

## Homework Equations

I know that when it crosses x axis y = 0
X3 - X2 - 20 =0
But don't know how to solve it from here

## The Attempt at a Solution

The marking scheme says 3.0 or 3.1
I couldn't get the answer even by substituting
I know I'm doing something stupid

## Answers and Replies

phinds
Science Advisor
Gold Member
Have you googled "how to solve a cubic equation" ?

The answer was there in the previous part
Thank you

Mark44
Mentor
@lioric, what's the complete problem? In the image you posted, there is no "n".

phinds
Science Advisor
Gold Member
@lioric, what's the complete problem? In the image you posted, there is no "n".
Look closer. Squint like Clint

#### Attachments

Last edited:
• berkeman
Mark44
Mentor
I was already squinting, looking at that unclear and incomplete image. By "there is no n" I meant that the equation doesn't involve n.

Ray Vickson
Science Advisor
Homework Helper
Dearly Missed
Look closer.

View attachment 229330
Squint like Clint
When I looked at it the first time (on my phone) there was no n. When I looked at it later (on my laptop) there was, magically, an n.

mathwonk
Science Advisor
Homework Helper
2020 Award
If the answer is desired as a decimal, it seems that it is an infinite decimal, hence cannot be written down exactly. Thus the simplest way to approximate this decimal to a few places is to just plug in values. when x=2, we get 8 - 4 -20 = -16. when x=3 we get 27 - 9 -20 = -2. so we re getting closer. when x = 4 we get 64 - 16 - 20 = 28, so we are getting further away. hence the answer lies between 3 and 4, so next try 3.1 and then 3.2, etc..., and stop when you get a positive result. then if say 3.1 gives a negative result and 3.2 gives a positive result, you can choose the one that gives a result closer to zero, as a first decimal place approximation.

• Charles Link
If the answer is desired as a decimal, it seems that it is an infinite decimal, hence cannot be written down exactly.

Just be careful with your language. 0.33333333333333333333... (to infinity) is also an infinite decimal, but can be written exactly as 1/3.