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Homework Help: Sum, Difference & Product Formulae

  1. Nov 8, 2012 #1
    My answer is almost correct, except for the negative sign. Can anyone help?

    Many thanks.

    1. The problem statement, all variables and given/known data

    Q. Without using a calculator, show that [itex]\sin10^{\circ}+\sin80^{\circ}=\sqrt{2}\cos35^o[/itex]

    2. Relevant equations

    3. The attempt at a solution

    [itex]2\sin(\frac{90}{2})\cos(\frac{-70}{2})[/itex]
    [itex]-2\sin45\cos35[/itex]
    [itex]-2(\frac{1}{\sqrt{2}})\cos35[/itex]
    [itex]\frac{-2}{\sqrt{2}}\cos35[/itex]
    [itex]\frac{-2\sqrt{2}}{2}\cos35[/itex]
    [itex]-\sqrt{2}\cos35[/itex]
     
  2. jcsd
  3. Nov 8, 2012 #2

    LCKurtz

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    Science Advisor
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    ##\cos(-35)=+\cos(35)##
     
  4. Nov 8, 2012 #3
    Thank you.
     
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