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Thread: Exact Value - Please Help?!

  1. #1
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    Exclamation Exact Value - Please Help?!

    Find the exact value of sin75, given that sin15 = (√3 -1)/2√2.

    Could someone please explain to me the relevance of sin15 in finding sin75?

    Thanx
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  2. #2
    Member Greengoblin's Avatar
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    presumably you are expected to know that $\displaystyle sin(60)=\frac{\sqrt{3}}{2}$ and $\displaystyle sin(\theta+\phi)=sin(\theta)cos(\phi)+cos(\theta)s in(\phi)$
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  3. #3
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    errr sori but that's made me more confused
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  4. #4
    Member Greengoblin's Avatar
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    Sorry, whats confusing you? often you are expected to memorize the sin and cosine of some comon angles, such as $\displaystyle 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 $\displaystyle cos(\theta)=\frac{adjacent}{hypotenuse}$
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  5. #5
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    oh i get it ...how stupid of me

    thanx
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