How do you solve cos13pi/12?
This is how I solve:
cos 13π/12
= cos (π/3 - π/4)
= cos(π/3)cos(π/4)
=cos(π/3)cos(π/4) + sin(π/3)sin(π/4) <I used the formula: cos(a-b)=cosacosb + sinasinb
=1/2 * 1/√2 + √3/2 * 1/√2
= (1+√3)/(2√2)
It says that the correct solution is: (-1-√3)/(2√2) *not rationalized*
Where do I put in negatives?
Update:OH. right. Thank you! =]
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Verified answer
Use the well-known fact that cos 13π/12 = -cos π/12. (13π/12 is in the third quadrant.)
What you have computed is cos π/12. Just put a minus sign in front and you have the correct answer.
You have found cos(Ï/12) instead of cos(13Ï/12).
cos(13Ï/12) = cos(Ï + Ï/3 - Ï/4)
= cos(Ï + (Ï/3 - Ï/4)) | use the formula: cos(Ï + a) = -cos(a)
= -cos(Ï/3 - Ï/4) etc.