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Math Help - Solving using sin(A+B) and cos rule

  1. #1
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    Solving using sin(A+B) and cos rule

    I need someone to solve this for me. I don't understand it so i'm hoping once i see how it solved i will be able to do the rest. Thanks a mill.

    1) Using the identities for sin(A+B) and cos^2(theta)+sin^2(theta)=1.

    Find sine 65 degrees, given that sin 40(degrees)=0.643 and sin 25(degrees)=0.423?
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  2. #2
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    You should know that

    \sin{(A+B)} = \sin{A}\cos{B} + \cos{A}\sin{B}.

    So \sin{65^{\circ}} = \sin{40^{\circ}}\cos{25^{\circ}} + \cos{40^{\circ}}\sin{25^{\circ}}.

    Use the Pythagorean Identity to find \cos{25^{\circ}} and \cos{40^{\circ}}...
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  3. #3
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    Thankyou so much. So do i now substitute the values in to replace the sin and cos.

    So (0.643 + 0.423) + (0.423 + ).643)

    ???
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  4. #4
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    You would if you had the correct values for \cos{25^{\circ}} and \cos{40^{\circ}}...

    Like I said, you have to use the Pythagorean Identity to find these values. Then you can substitute your 4 values into the sum rule.
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  5. #5
    Senior Member eumyang's Avatar
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    Why on earth did you post this question for the third time?

    http://www.mathhelpforum.com/math-he...em-153105.html

    http://www.mathhelpforum.com/math-he...em-153216.html

    Are people not allowed to bump their own threads or something!?!?
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