CBSE Class 11-science Answered
7th sum plz
Asked by lovemaan5500 | 09 Oct, 2018, 06:38: AM
Expert Answer
(cos 18 + i sin18)(cos 54 + isin54)(cos 108 + i sin108)
= (cos 18 + i sin18)(cos 54 + isin54) (-sin18 + i cos 18) cos (90 + x) = -sinx and sin(90 + x) = cos x
= (cos 18 + i sin18)(-sin18 + i cos 18)(cos 54 + i sin54)
= (-cos 18 sin 18 + i cos2 18 - i sin2 18 + i2 sin 18 cos 18)(cos 54 + i sin54)
= [-cos 18 sin 18 + i (cos2 18 - sin2 18) - sin 18 cos 18](cos 54 + i sin54)
= (-2sin 18 cos 18 + i cos 36)(cos 54 + i sin54)
= (-sin 36 + i cos 36)(cos 54 + i sin54)
= - (sin 36 - i cos 36)(sin 36 + i cos 36)
= - (sin2 36 - i2 cos2 36)
= - (sin2 36 + cos2 36)
= - 1
Answered by Sneha shidid | 12 Oct, 2018, 03:36: PM
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