Absolute value

I spent a fair bit of time thinking about absolute value problems in the form |x+a| - |x-b| = c. Many students were struggling with visualising what these functions actually look like. What was happening when we try and solve them?

For example:

|x+5| - |x - 2| = 6

How could I display this equation graphically to give students an understanding of the underlying algebra to solve it?

I tried graphing y = |x+5| - |x - 2| and y=6 to find the intersection but was unsatisfied with the result as y = |x+5| - |x - 2| is not something easily tied to the absolute value concept or 'v' shaped absolute value graphs.

I was eventually satisfied with graphing y= |x+5| and y = |x-2| and then examining each part of the graph until I found a section of the graph that was 6 units apart.

For those wondering how to put it into a graphics calculator while exploring the concept

Go Menu -> Graph & Tab

Edit -> Clear All -> ok

at "Y1:" ->Softkeyboard->mth tab->select 'x'->type "x+5)" (it will change from abs(x+5) to |x+5|)

at "Y2:"->select 'x'->type "x-2)"

ensure that the boxes next to "Y1" and "Y2" are ticked

Now the temptation is to assume the answer is the intersection point.

but if we look at the equation |x+5| - |x - 2| = 6, it is asking "for what value of x is the value of |x+5| (the dotted line) subtract the value of |x-2| (the solid line) equal to 6". When is the gap between the two functions +6.

We can ignore values of x<= -5 as y=|x+5| is below y=|x-2| and the subtraction will only give negative values (we are looking for a gap of +6 which is a positive value).

We can also ignore values up to the intersection point as this also will only result in negative values.

The next place I looked is at x>=2 as the gap is constant and positive after this point (both functions have the same gradient).

at x=2, |x+5| is equal to 7 and |x-2| is equal to 0. |2+5| - |2-2| = 7. We can ignore values where x>=2 as the answer is not +6.

In fact the only possible solution has to lie between the intersection point (x~-1.5) and 2 and is probably closer to 2.

For y=|x+5| all values are positive between -1.5 < x < 2

For y=|x-2| all values are negative between -1.5 < x < 2

To ensure positive values for x-2 in the range -1.5< x < 2 we need to take the negative of (x-2) when solving the equation |x+5| - |x - 2| = 6.

x+5 - (-(x-2)) = 6
2x+3 = 6
x=1.5

Check answer:

|x+5| - |x - 2| = 6
Let x=1.5
|1.5+5| - |1.5-2| = 6
LHS =| 6.5| -| -.5|

= 6.5 - 0.5
= 6.0
= RHS

Viola.

It would also be interesting to explore |x+a| + |x-b| = c,  |x+a| = |x-b| and -|x+a| = c in a similar way.

Here is a link to other CAS calculator posts.

Keeping up and reducing doubt.

It's a hard ask keeping up sometimes. It's my first year teaching year 12 Calculus, Probability and Statistics. Your focus slips from your good year 12 kids, to the lower classes where your interest lies and all of us sudden you are faced with a crisis of confidence.

Are you good enough? Have you done enough? In a subject like maths, students need you to always be on the ball, or their confidence also suffers.

It's times like that you have to go back to basics. Do each exercise. Talk to a staff member that you trust. Get your confidence back. Maybe put some things aside for awhile.

I remember when I found out that I was on a pathway to take these classes, I wondered if I was up to the challenge, if my mathematics had risen back to that level. I argued that these kids needed the best teacher available. I still believe this should happen, but will fall in line with department wishes.

Maybe I have to rekindle some doubt in myself and do some real work to improve. It's a shame, because I'm really making some ground putting effort into my teaching capability with the lower classes. My masters research is teaching me a lot about myself and my teaching style - a teaching style that is much harder to work on with a good bunch of kids that will respond to a simple instruction.

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Limited trial of National Curriculum.

Again, on a rushed timetable, the government pushes out information that a trial is to be done this year (5 weeks into term). Teachers will be given programmes (at the 11th hour) and the kids will have to deal with a poorly understood curriculum by teachers through no fault of the teachers themselves.

Successful project management is not rushed and has an understanding of as many factors as possible. Head-in-the-sand management is a recipe for disaster. Success becomes a factor of luck rather than good management. Children's futures should not be a part of a recipe for the re-election of the Labor party at the next election. It should be a bipartisan agreement implemented with long term planning and proven methods.

Regardless of any issues with the trial, the national curriculum will be rolled out next year. What is of bigger concern is that senior school curriculum will be rolled out later this year. I really hope senior school curriculum will be given more consideration than the lower school programme as the consequences for university entrance and TAFE integration are far more severe than upper school teachers coping with students who have suffered a partial implementation with gaps in learning.

Theory and practical application are two completely different beasts. To quote that 900 people have been involved in the theoretical design of the curriculum (with little coalface application) is not going to impress. Are these the same 900 people that designed and implemented OBE in WA? I really hope not.

The media release is found here.