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Thread: Exact Value - Please Help?!

  1. October 7th 2008, 12:11 AM #1
    xwrathbringerx
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    Exclamation Exact Value - Please Help?!

    Find the exact value of sin²75°, given that sin15° = (√3 -1)/2√2.

    Could someone please explain to me the relevance of sin15° in finding sin²75°?

    Thanx
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  2. October 7th 2008, 12:18 AM #2
    Greengoblin
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    presumably you are expected to know that sin(60)=\frac{\sqrt{3}}{2} and sin(\theta+\phi)=sin(\theta)cos(\phi)+cos(\theta)s in(\phi)
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  3. October 7th 2008, 12:20 AM #3
    xwrathbringerx
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    errr sori but that's made me more confused
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  4. October 7th 2008, 01:08 AM #4
    Greengoblin
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    Sorry, whats confusing you? often you are expected to memorize the sin and cosine of some comon angles, such as 30^{\circ},45^{\circ},60^{\circ}, etc.

    The fomula is for the sine of the sum of two angles. Have you seen this yet? I just realised you don't have the cosines of the angles anyway, so probably you have to draw a diagram, label the sides of the triangles, and remember cos(\theta)=\frac{adjacent}{hypotenuse}
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  5. October 7th 2008, 01:13 AM #5
    xwrathbringerx
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    oh i get it ...how stupid of me

    thanx
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