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# Remembering Trig Identities watch

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1. Does anyone have ways of remembering trig identities? Especially double angle formulae (sin2A = 2sinAcosA , cos2A =......)
2. (Original post by hms30)
Does anyone have ways of remembering trig identities? Especially double angle formulae (sin2A = 2sinAcosA , cos2A =......)
Not really. There's only 3 formulae to remember and they're probably in your formula book(unless you're talking about the hyperbolic ones too, in which case you can easily work them out from the relevant circular ones using eg. Osborne's rule).
I wouldn't remember the ones for sin2x and cos2x, just the ones for sin(A+B), cos(A+B), tan(A+B) and just derive anything more specific when you need it.
3. I don't know if this will help but when I was preparing for my engineering entrance there was a ton of formulas in math Chem and physics. Initially I tried to mug it up but it didn't work. The best way I found was to do problems based on that formula and after a while I started remembering them. Also make a formula sheet and go through it once a week.
4. (Original post by morgan8002)
Not really. There's only 3 formulae to remember and they're probably in your formula book(unless you're talking about the hyperbolic ones too, in which case you can easily work them out from the relevant circular ones using eg. Osborne's rule).

I wouldn't remember the ones for sin2x and cos2x, just the ones for sin(A+B), cos(A+B), tan(A+B) and just derive anything more specific when you need it.
Your method is fine but during competitive exams even seconds are precious and a lot of time is lost in deriving 3x or 4x formulas. Deriving should be the last option.
5. (Original post by a2874)
Your method is fine but during competitive exams even seconds are precious and a lot of time is lost in deriving 3x or 4x formulas. Deriving should be the last option.
It's not worth remembering more formulae than you need to. There are infinitely many multi-angle formulae so where do you draw the line?

If time is an issue and you need to [remember or derive] cos(nx), sin(nx) or tan(nx) for n>2 formulae quickly in an exam, I recommend learning de Moivre's theorem. You can reasonably quickly and easily derive the formula for any reasonably sized integer n. Depending on the exam, if you make a mistake remembering it you might get no or very few marks, but if you make a mistake deriving it you'll probably get most of the marks. I had to derive tan(9x) in a mock last year and there's no way I could have remembered it or derived it using double angle formulae at the pace of the exam.
The hyperbolic ones can also be worked out via de Moivre's theorem or Obsorne's rule.
6. Practice loads of questions and you'll be fine.
7. I think I will try practicing more questions and if not I will try deriving them. Thanks everyone!
8. you could maybe make them the words to your favorite "pop" song and dance around singing ?
9. I wrote the hard ones on my calculator and palm.

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Updated: November 13, 2015
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