SOLUTION: assuming that cos(alpha)=-6/7 with alpha between pi/2 and pi and that cos(beta)=8/9 with beta between 3pi/2 and 2pi, evaluate cos(alpha+beta) exactly.

Algebra ->  Trigonometry-basics -> SOLUTION: assuming that cos(alpha)=-6/7 with alpha between pi/2 and pi and that cos(beta)=8/9 with beta between 3pi/2 and 2pi, evaluate cos(alpha+beta) exactly.      Log On


   

Question 1052903: assuming that cos(alpha)=-6/7 with alpha between pi/2 and pi and that cos(beta)=8/9 with beta between 3pi/2 and 2pi, evaluate cos(alpha+beta) exactly.
Answer by solver91311(24713) About Me  (Show Source):
You can put this solution on YOUR website!










But is positive in QII, so



Similarly





So:


I'll leave it to you to simplify.

John

My calculator said it, I believe it, that settles it