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

[goldenroll]

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)

 

[royhaas]

Re: Addition and Subtraction Formula Trig

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

 

[Loren]

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)