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# Remembering Double Angle formulae Watch

1. Anybody know a way to remember double angle formulae after substitution? I know how to work them out but if i can remember that you make it a lot easier. Something for atleast the 2 equations I have *ed.
sin 2x = 2sin x cos x
cos 2x = cos2x -sin2x
cos 2x = 2cos2x -1 *
cos 2x = 1 - 2cos2x *
tan 2x = 2tan x/ 1-tan2x
2. (Original post by Pink kitten)
Anybody know a way to remember double angle formulae after substitution? I know how to work them out but if i can remember that you make it a lot easier. Something for atleast the 2 equations I have *ed.
sin 2x = 2sin x cos x
cos 2x = cos2x -sin2x
cos 2x = 2cos2x -1 *
cos 2x = 1 - 2cos2x *
tan 2x = 2tan x/ 1-tan2x
Practice using them and they become second nature.
3. (Original post by Pink kitten)
Anybody know a way to remember double angle formulae after substitution? I know how to work them out but if i can remember that you make it a lot easier. Something for atleast the 2 equations I have *ed.
sin 2x = 2sin x cos x
cos 2x = cos2x -sin2x
cos 2x = 2cos2x -1 *
cos 2x = 1 - 2cos2x *
tan 2x = 2tan x/ 1-tan2x
As above, practice, practice, practice!
and the secret ingredient...
Spoiler:
Show
practice!

then you will have no trouble remembering them

There are plenty of exercises in textbooks to work from.
4. (Original post by Pink kitten)
Anybody know a way to remember double angle formulae after substitution? I know how to work them out but if i can remember that you make it a lot easier. Something for atleast the 2 equations I have *ed.
sin 2x = 2sin x cos x
cos 2x = cos2x -sin2x
cos 2x = 2cos2x -1 *
cos 2x = 1 - 2cos2x *
tan 2x = 2tan x/ 1-tan2x
For the two you have *ed it takes a matter of seconds to arrive at them from the one above ( using sin^2x + cos^2x = 1), so it is not necessary to remember them. As for the rest, as the others said, do loads of questions involving them!

Alternatively deriving them a few times can help.
5. (Original post by Pink kitten)
Anybody know a way to remember double angle formulae after substitution? I know how to work them out but if i can remember that you make it a lot easier. Something for atleast the 2 equations I have *ed.
sin 2x = 2sin x cos x
cos 2x = cos2x -sin2x
cos 2x = 2cos2x -1 *
cos 2x = 1 - 2cos2x *
tan 2x = 2tan x/ 1-tan2x
You can derive them in like 10s off the formula sheet anyway since you have sin(a+b) cos (a+b) on there, but when you do a lot of past papers they become pretty much second nature anyway

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Updated: June 1, 2016
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