Prove:
cos 20 + cos 60 + cos 100 + cos 140 = 1 / 2
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Answers
Answered by
10
Step-by-step explanation:
cos 20+cos 60=2cos(20+100/2)cos(100-20/2)
=2cos60cos40
2 1/2 cos40==cos 40
cos 20+ cos60+cos 100+ cos 140
cos 40+ cos 60+ cos 140
cos 20+1/2+(-cos40)
1/2...
clear......
Answered by
13
cos 20+cos 60=2cos(20+100/2)cos(100-20/2)
=2cos60cos40
2 1/2 cos40==cos 40
cos 20+ cos60+cos 100+ cos 140
cos 40+ cos 60+ cos 140
cos 20+1/2+(-cos40)
1/2
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