A community for students.

Here's the question you clicked on:

55 members online
  • 0 replying
  • 0 viewing

nincompoop

  • one year ago

what is not correct about cross multiplication?

  • This Question is Closed
  1. Empty
    • one year ago
    Best Response
    You've already chosen the best response.
    Medals 1

    as opposed to dot multiplication? :P

  2. nincompoop
    • one year ago
    Best Response
    You've already chosen the best response.
    Medals 2

    lollll

  3. nincompoop
    • one year ago
    Best Response
    You've already chosen the best response.
    Medals 2

    okay, we teach or use the phrase cross-multiply without really showing why it works and even tho it appears to violate one of the key rules in maintaining an equation.

  4. Empty
    • one year ago
    Best Response
    You've already chosen the best response.
    Medals 1

    honestly I don't even know what cross multiplication is anymore, could you explain it

  5. nincompoop
    • one year ago
    Best Response
    You've already chosen the best response.
    Medals 2

    same here... HAHA but here's the gist that I have so far gathered.

  6. nincompoop
    • one year ago
    Best Response
    You've already chosen the best response.
    Medals 2

    |dw:1440547770891:dw|

  7. nincompoop
    • one year ago
    Best Response
    You've already chosen the best response.
    Medals 2

    we have this fundamental rule about keeping an equation by performing the same operation we did on one side to the other side

  8. nincompoop
    • one year ago
    Best Response
    You've already chosen the best response.
    Medals 2

    |dw:1440547862839:dw|

  9. nincompoop
    • one year ago
    Best Response
    You've already chosen the best response.
    Medals 2

    it seems trivial at first, but I think this minor thing compounds many problems.

  10. Empty
    • one year ago
    Best Response
    You've already chosen the best response.
    Medals 1

    yeah I agree, it's teaching pattern matching for no reason, just like FOIL sucks. It's just distributing. Anyone who can see 2*3=6 should be able to see (1+1)*(1+2)=6 will have to be true too.

  11. Empty
    • one year ago
    Best Response
    You've already chosen the best response.
    Medals 1

    yeah, probably why I forgot what cross multiplying is because it's literally a waste of a concept haha

  12. nincompoop
    • one year ago
    Best Response
    You've already chosen the best response.
    Medals 2

    I was thinking about this a while back and then I stumbled on a problem that just reminded me that this needs to be addressed. http://openstudy.com/study#/updates/55dcf68ee4b03aeb8dc1a352

  13. anonymous
    • one year ago
    Best Response
    You've already chosen the best response.
    Medals 0

    The main issue I would foresee with it is in some cases like for the equation \(\dfrac{1}{x}=\cdots\), cross-multiplying can mislead you into thinking that that \(x=0\) might be a valid solution, which isn't good for obvious reasons.

  14. nincompoop
    • one year ago
    Best Response
    You've already chosen the best response.
    Medals 2

    correct

  15. anonymous
    • one year ago
    Best Response
    You've already chosen the best response.
    Medals 0

    I suppose there's no issue if you're dealing with concrete rational numbers, and perhaps it's useful as a sort of mnemonic device. You just have to be careful (as anyone doing math should be).

  16. anonymous
    • one year ago
    Best Response
    You've already chosen the best response.
    Medals 0

    cross multiplication is literally how you define rational numbers usually

  17. anonymous
    • one year ago
    Best Response
    You've already chosen the best response.
    Medals 0

    a rational number is an element of the quotient of \(\mathbb{Z}^2\) by the equivalence relation \((a,b)\sim(c,d)\Leftrightarrow ad-bc=0\)

  18. anonymous
    • one year ago
    Best Response
    You've already chosen the best response.
    Medals 0

    so \(\mathbb Q=\mathbb Z/\sim\) and we denote the equivalence class \([(a,b)]\in\mathbb Q\) as \(\frac{a}b\)

  19. Not the answer you are looking for?
    Search for more explanations.

    • Attachments:

Ask your own question

Sign Up
Find more explanations on OpenStudy
Privacy Policy

Your question is ready. Sign up for free to start getting answers.

spraguer (Moderator)
5 → View Detailed Profile

is replying to Can someone tell me what button the professor is hitting...

23

  • Teamwork 19 Teammate
  • Problem Solving 19 Hero
  • You have blocked this person.
  • ✔ You're a fan Checking fan status...

Thanks for being so helpful in mathematics. If you are getting quality help, make sure you spread the word about OpenStudy.

This is the testimonial you wrote.
You haven't written a testimonial for Owlfred.