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Teaching changing the subject

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

  1. EmmaLVB

    EmmaLVB New 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!
  2. mature_maths_trainee

    mature_maths_trainee New 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?
  3. PaulDG

    PaulDG Occasional commenter

    Do they understand balancing?

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

    bombaysapphire Star 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. cybola - metaphorically [​IMG]
  6. 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. 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. Emma, I would use flow diagrams, as long as subject appears once it works fine.
  9. strawbs

    strawbs Established commenter

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

    Nazard New 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.

  11. Slinkywhippet

    Slinkywhippet New commenter

    Students need to realise the point of rearranging a formula. I start with numbers - Eg 'Find me all the sums you can linking 12, 2 and 6'. Then I move onto 'if a + b = c, find me a formula starting b ='. I also get them to realise the point of changing the subject by relating it to formulas they know such as s = d/t. They appreciate that the question will not always ask them do find the speed. If this is done, I have found students understand why it is important to be able to rearrange formulas and are able to relate it to the balancing method.
  12. I started with balancing, then usuing 2 mini whiteboards -then hit on veggie haribos. got their attention and the visual effect seemed to work.
    the yr 7,8,9 and 10's remembered enough to get marks in their end of unit tests.

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