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# C3 Trig watch

1. Need help with this.

Don't get it at all.
2. (Original post by uer23)
Need help with this.

Don't get it at all.
Its known as the half angle formulae ,,
cos2A=1-2sin^2A-> a result you should know
replace A by A/2
and rearrange
you will find what youre looking for..
3. cos2x = 1 - 2sin^2(x)

so from that identity
divide by 2:

(1/2)cos2x = 1/2 - sin^2(x)

and then divide the angle by 2:

(1/2)cosx = 1/2 - sin^2(x/2)

and then do a bit of rearranging:

sin^2(x/2) = (1/2) - (1/2)cosx

=sin^2(x/2) = 1/2(1 - cosx)
4. (Original post by rbnphlp)
Its known as the half angle formulae ,,
cos2A=1-2sin^2A-> a result you should know
replace A by A/2
and rearrange
you will find what youre looking for..
Now I'm even more confused

I don't see where you get the cos2A from.
5. (Original post by uer23)
Now I'm even more confused

I don't see where you get the cos2A from.
Its a identity you need to use , to get to the desired result..
you cant easily start from sin^2(x/2) and end up with RHS ..

so you look at the equation given
Cos2A=1-2sin^2A seems to be the best candidate , simply by replacing A by A/2 , and rearranging the result follows

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