Question
The value of cos² 10° + cos² 20° + cos² 30° +
……. + cos² 90° isSolution
ATQ, We know cos(90° - A) = sin A cos² 10 + cos² 20 + cos² 30° +……. + cos² 90° = cos² 1 + cos² 20 + cos² 30° + cos² 40° + sin² 40+ sin² 30° + sin² 20 + sin² 10+ cos² 90 = (sın² 10° + cos2 10) + (sin² 20 + cos² 20) + (sin² 30° + cos² 30) + (sin² 40+ cos² 40°) + cos² 90 = 1+1+1+1+0 = 4
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