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2014-05-16T11:25:59+07:00
Cos2(150) + cos2 (350) + cos2 (550)+ cos2 (750) = cos 300+cos700+cos1100 +cos1500
= cos 300 + cos 340 + cos 20 + cos 60
= Cos(360-60) + Cos(360 - 20) + cos 20 + cos 60
= cos60 + Cos 20 + cos 20 + cos 60
= 1/2 + 2cos20 + 1/2
= 1 + 2cos20
= 1 + 0,9397
= 1,9397
≈ 1,94
≈ 2
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