Calculus

I had this on a test:

Use the half-angle formula to find the exact value of the expresseion:
sin 22.5 degrees

my answer - 1/2 sqrt2 - sqrt 2 was wrong

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  1. 22.5 is half of 45º, those trig ratios are known
    using cos 2x = 1 - 2sin^2 x
    and knowing that cos 45º = √2/2

    cos 45º = 1 - 2 sin^2 22.5º
    √2/2 = 1 - 2sin^2 22.5
    sin^2 22.5 = (1-√2/2)/2

    sin 22.5 = √[(1-√2/2)/2]

    This probably could be simplified a bit, but it is the "exact" answer

    ( you should have realized that sin 22.5 could not possibly be negative, like your answer is
    Why didn't you use your calculator to check your answer?)

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  2. cos45= - sin^2 22.5 +cos^2 22.5
    =1 - 2sin^2 22.5

    sin 22.5= sqrt (-1/2 cos45 +1/2 )
    = sqrt (1/2 - 1/(2sqrt2) )

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    bobpursley
  3. I was thinking clearly, silly mistake that's for sure , thanks

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