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Thread: Addition and Subtraction Formula Trig: evaluate sin(255),

  1. #1

    Addition and Subtraction Formula Trig: evaluate sin(255),

    I'm having trouble on these problems and their due tonight, its the only problems I have left on my homework if anyone can help on any of them it will be much appreciated.

    Find the area of a triangle that has the following dimensions. Give your answer correct to the tenth place.
    sides of length 6 and 9, and included angle 76.

    Use an addition or subtraction formula to find the exact value of the expression.

    Sin (255)
    Tan (-195)
    Tan (-5pie/12)

    cos (80) cos (10) - sin (80) sin (10)
    I keep getting 0 and 1 for this problem but its wrong.

    cos (7pie/6) cos (-pie/2)- sin (7pie/6) sin (-pie/2)

  2. #2
    Full Member
    Join Date
    Dec 2005
    Location
    San Antonio,Texas
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    Re: Addition and Subtraction Formula Trig

    Perhaps you should start with the cosine law with the triangle.
    255=360-(60+45)
    195 = 180+15
    [tex]5\pi/12 = \pi/4+\pi/6[/tex]
    90=80+10
    2/3 = 7/6 - 1/2

  3. #3
    Senior Member
    Join Date
    Aug 2007
    Posts
    1,303

    Re: Addition and Subtraction Formula Trig

    After using the Law of Cosines to find the length of the third side, use Hero's formula for its area.

    sin 255 = sin(300-45)
    tan(-195) = tan (-150-45)

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