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An unfortunate choice of numbers?

Discussion in 'Mathematics' started by gainly, May 30, 2020.

  1. gainly

    gainly Star commenter

    I was doing this question from an MEI past paper with a student.

    Solve: ln(2+e^x) = 2x

    I was surprised when he said almost immediately x = ln2, which of course is correct.

    Since I'm having to do lessons online I couldn't see his working so asked how he got that.

    His method, ln2 + x = 2x
    so x = ln2

    So a complete misunderstanding of logs but it gives the right answer.

    If they'd had for example, Solve: ln(6+e^x) = 2x, this couldn't have happened.
    Piranha likes this.
  2. nervousned

    nervousned Senior commenter

    But does it matter? Without correct working they surely wouldn't have got the marks? Never marked for MEI but they wouldn't have on any other paper I've marked. The possible error would have been pointed out too.
  3. gainly

    gainly Star commenter

    You're probably right. Students do sometimes ask me to go through tests with them that they've done at school and a few times I've found questions where they have got the right answer by a fluke and had it marked right even though their working was completely wrong. Hopefully this wouldn't happen in an actual A level exam.
  4. nervousned

    nervousned Senior commenter

    I remember one of my students had his paper back and complained to me that he had got no marks out of six for a question where he had the correct final answer. His working was complete nonsense. It was bizarre that he had somehow even got anywhere near the actual answer. Needless to say, I told him that it had been marked correctly. He sulked about it for a while though.
    Lee Fields likes this.
  5. jcstev

    jcstev New commenter

    General exam marking guidelines: unless the questions says "show your working" then the correct answer with no workings scores full marks; correct answer derived fortuitously from incorrect workings scores no marks.
  6. gainly

    gainly Star commenter

    Using the student's wrong method he could actually have got the right answer without showing any working, so on that basis could have got full marks.
  7. nervousned

    nervousned Senior commenter

    This is not true at A level unless it is reasonable to do the question without working. This is not the case for @gainly example.
  8. adamcreen

    adamcreen Occasional commenter

    NOPE. That's not how marking works in the UK.
    nervousned likes this.
  9. Piranha

    Piranha Star commenter

    Usually true. From an MEI mark scheme, dated 2019.

    A Accuracy mark, awarded for a correct answer or intermediate step correctly obtained. Accuracy marks cannot be given unless the associated Method mark is earned (or implied). Therefore M0 A1 cannot ever be awarded.

    However, the mark schemes for some questions specified that an unsupported answer could earn full marks. But an answer arising from clearly wrong working would not be included in this. I doubt if this particular question would allocate marks for a correct answer with no working, but that probably would have been clear from the mark scheme. I say "probably|" because there might have bene a ruling on the subject made after the mark scheme had been finalised, which only people involved in marking that paper would know about.
  10. Alexdiver

    Alexdiver New commenter

    There was about 3 instances of this on 1MA1 3H edexcel last year - especially the histogram/pie chart question

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