You are Here: Home >< Maths

C3 help!!!! watch

1. for graphing y=f(IxI)
why is it a reflection in the y axis, i thought the makes all X values positive and that means Y values too????
2. (Original post by JustgotAstar)
for graphing y=f(IxI)
why is it a reflection in the y axis, i thought the makes all X values positive and that means Y values too????
The y value of the negative x values becomes positive. By drawing this part of the graph with the positive parts, it looks like it's a reflection in the y axis.
It's just a linear function that meets the x axis at y=0 so the reflection is in y=0, the y axis.

Spoiler:
Show

That's what I understand.

3. If you take f(x) = x that will mean y = |x|
So the y value when x = -2 will be the same as when x = 2 so it is going to be reflected in the y axis

However if f(x) = x-4 that will mean y = |x|-4 so values where |x|<4 the y value will be negative, so it doesn't always follow that y = f(|x|) produces positive values of y, it depends of the function f(x)
4. The X value can be a negative, however the modulus of a plus or minus value is always positive i.e so when X = -3, Y = 3 as the -3 is converted into a 3, so the Y value never goes below 0.
5. (Original post by JustgotAstar)
for graphing y=f(IxI)
why is it a reflection in the y axis, i thought the makes all X values positive and that means Y values too????
Try plotting the simplest function: y = x and then plot y = modulus x. See what happens!
6. The value of y will be positive regardless of |x| being positive or negative because the modulus function states that the value will be positive hence the positive y values. I just remember it like this because it makes it easier to understand and apply unless you are doing complicated problems.
7. (Original post by JustgotAstar)
for graphing y=f(IxI)
why is it a reflection in the y axis, i thought the makes all X values positive and that means Y values too????
For a function , replacing means that and , and so on until . In fact, . This is symmetrical around the line , which is the y-axis and hence the portion looks reflected in the y-axis to look like a reflection on portion of the graph.

For all OUTPUTS to be positive, ie your Y values as you'd refer to them, you'd need to have or something like that with the modulus brackets on the entire function.

Simple example, take which takes strictly negative values for , then graphing shows you the reflection of the portion of in the y-axis, and so the graph must dip again below 0 in the region
8. Wow, thanks guys, you all made it really clear particularly RDKGames

Related university courses

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: October 23, 2017
Today on TSR

My dream is to become a WAG

I wont settle for anything else!

Poll
Useful resources

Maths Forum posting guidelines

Not sure where to post? Read the updated guidelines here

How to use LaTex

Writing equations the easy way

Study habits of A* students

Top tips from students who have already aced their exams

Chat with other maths applicants