SOLUTION: Use an Addition or Subtraction Formula to write the expression as a trigonometric function of one number. cos(4pi/7)cos(5pi/21)+ sin(4pi/7)sin(5pi/21) = Find its exact value.

Algebra ->  Trigonometry-basics -> SOLUTION: Use an Addition or Subtraction Formula to write the expression as a trigonometric function of one number. cos(4pi/7)cos(5pi/21)+ sin(4pi/7)sin(5pi/21) = Find its exact value.       Log On


   

Question 924665: Use an Addition or Subtraction Formula to write the expression as a trigonometric function of one number.
cos(4pi/7)cos(5pi/21)+ sin(4pi/7)sin(5pi/21) =
Find its exact value.
cos(4pi/7)cos(5pi/21)+ sin(4pi/7)sin(5pi/21) =

Please explain
Thank you
Answer by Alan3354(69443) About Me  (Show Source):
You can put this solution on YOUR website!
Use an Addition or Subtraction Formula to write the expression as a trigonometric function of one number.
cos(4pi/7)cos(5pi/21)+ sin(4pi/7)sin(5pi/21) =
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Look up the formula for cos(A-B)