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(a Take a Test-fatima carrillo Mozilla Firefox O https//www.mathxl.com/Student/PlayerTest.aspx?testld-1452568108icenterwin yes Math 1316, Fall 2016 Test: Chapter 5 Test, Sections 5.1 through 5.6 This Question: 1 pt 5 of 15 (5 complete) This Test: 15 pts poss Use the product-to-sum identities to rewrite the expression 2cos COS E Which choice below is equivalen to 2cos 4 COS 5 o 9 A. cos co 20 O B 2 cos t cos 20 20 20 O C. O D COS t cos 20 20 sin 20 Sin n Click to select your answer javascript doExercise(5);

Solution

2cos(pi/4)cos(pi/5)

cos A + cos B= 2cos(A +B)/2 cos (A-B)/2

on comparing them we get

A+B/2=pi/4 and A-B/2=pi/5

And on solving these equations we get

A=9pi/20 and B=pi/20

therefore we get

cos (9pi/20) + cos (pi/20)

Therefore the correct option is C.