Expert: Sherman D. - 1/5/2005

Question
hello,

my questions is:

If sin (insert beta symbol here)= 5/13 and cos (insert theta symbol here) = 4/5, find cos (beta + theta)

I used the sin and cos info to make a right triangle to figure out all the measurements of the signs and when i work it out it does not equal what my books says it does. I know my answer is wrong because it turns out to be 0.0072, when all the other answers do not even come close to being like this decimal number.

Thanks,

Chase

Answer
sinA = (5/13)
cosB = (4/5)
cos(A + B) = x

inverse sin on both sides
A = sin^-1(5/13)

inverse cos on both sides
B = cos^-1(4/5)

cos((sin^-1(5/13)) + (cos^-1(4/5))) = .50769230 with the 769230 repeating or (33/65)

Also using
cos(A + B) = (cosA * cosB) - (sinA * sinB)
cos(sin^-1(5/13))*(4/5) - (5/13)(sin(cos^-1(4/5)))
(33/65)

found at
www-physics.ucsd.edu/~aklohs/studies/math/trig.html

Let me know if this is the answer your book has.

Your answer maybe in Radians, i think it would be about .008860902

I didn't know if you we saying the answer is suppose to be .0072 or if you are saying you got an answer of .0072.