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Cos20+cos40+cos60-4cos10cos20cos30 equals to option (a) -1 option b -3/4 option c 1 option d 0

Cos20+cos40+cos60-4cos10cos20cos30 equals to option (a) -1 option b -3/4 option c 1 option d 0

Grade:11

2 Answers

poonam
18 Points
5 years ago
Option a -1  first apply cosc+cosd on cos20+cos40 and then apply 2cosacosb on cos10cos20 solve It you will answer -1
mkmjnj
43 Points
5 years ago
let us solve this question in parts
  •    cos20 + cos40 = 2cos30cos10 
  •   4cos10cos20cos30 = 2[ 2cos10cos20 ]cos30  =  2 [cos30 + cos10 ] cos30   =  2cos30cos30 + 2cos10cos30
  •   cos60 = ½
now solve them by putting these values in question
you will get ANS = 1

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