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    Need help with this.



    Don't get it at all.
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    (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..
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    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)
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    (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 :confused:

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