# Teaching changing the subject

Discussion in 'Mathematics' started by EmmaLVB, May 11, 2012.

1. ### EmmaLVBNew commenter

Hi guys,
I have started to teach change the subject to my year 9 set 3 and they just don't understand it. They can solve equations fine but this is a big problem for them. Does anyone have any tips on hhow they go about teaching it/explaining it? Any interesting sites that help. Any ideas would be greatfully received!
Thanks
Emma

2. ### mature_maths_traineeNew commenter

I haven't tried this, but how about just 'changing the subject' with pure numbers (no algebra) first of all. What I mean is, create a piece of arithmetic, such as '4 = (8x2) -12', and then without calculating or changing any of the numbers themselves, get them to re-arrange it into the form '2 = ...' or '8 = ...'. e.g. '2 = (4 + 12) / 8'. At least this might establish the concept, without the possible distraction of the algebra itself?
MMT

3. ### PaulDGOccasional commenter

Do they understand balancing?

(If they solve everything with "story of x", I think you're doomed.)

4. ### bombaysapphireStar commenter

Good point. My first suggestion in post 3 assumes that they solve equations using the balance method.
If not then they need to be taught this method before covering changing the subject. Although if they are learning changing the subject then surely they can solve an equation with the unknown on both sides?

5. ### florapost

cybola - metaphorically

6. ### cyolba

Cheers, flora. I could blame a sticky keyboard, but I was actually just too lazy to proof-read my post.
.
cyolba, knot pruf wreeding thiz won eyethere

7. ### charliedontsurf

Start with 1-step: y = x-1. y = x +3, y = 2x, y =x/2, changing the letters and numbers so they do loads.
Move to 2-step: y = 2x + 1, y = x/3 -1 use of brackets, do loads.
Move to 3 step: y = 3x^2 + 4, y = rt(x/2 + 3)
The whole thing might be seen as a game. You DO THE OPPOSITE to leave poor little x all on her own.

8. ### rmsia

Emma, I would use flow diagrams, as long as subject appears once it works fine.

9. ### strawbsEstablished commenter

I wouldn't teach it to my year 9 set 3!!

10. ### NazardNew commenter

My way of doing this is similar to the idea suggested by MMT earlier in the thread.

I get them to solve some equations that involve balancing. I then ask them to redo these, but without simplifying at each stage. For example:

4x + 1 = 13 leads to

4x = 13 - 1, which leads to

x = (13 - 1) / 4 [but written as a fraction].

Then they can compare this to what they need to do to make x the subject of ax + b = c

The issue that I think many pupils have is that they want there to be "an answer". They are happy when x = 3 because this feels like an answer, but x = (c-b)/a feels unfinished and incomplete.

HTH