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# 1 - cos2x

1. is

1 - cos2x equal to (sinx)^2

??????thanks
2. Nearly. It's equal to 2(sinx)^2. Proof is from the double angle formula.
3. No

Rule is

cos^2 x + sin^2 x =1
4. (Original post by tommm)
Nearly. It's equal to 2(sinx)^2. Proof is from the double angle formula.
thanks.

is that just one of the ones we have to remember?
5. (Original post by rye225)
No

Rule is

cos^2 x + sin^2 x =1
You're thinking of the wrong rule; the rule you want is the double angle formula for cosine.

(Original post by sonic23)
thanks.

is that just one of the ones we have to remember?
You're given , so you can derive and play with it until you get it in the right form... but it's worth remembering these three, if nothing else:

Then you can rearrange it.
6. (Original post by sonic23)
thanks.

is that just one of the ones we have to remember?
Pretty much, yes. As an experienced A-leveller, I can immediately recall , but that's because I've used it so many times. Practice makes perfect.
7. (Original post by nuodai)
You're thinking of the wrong rule; the rule you want is the double angle formula for cosine.

You're given , so you can derive and play with it until you get it in the right form... but it's worth remembering these three, if nothing else:

Then you can rearrange it.

thanks
8. Not a problem.

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