求cos7`+cos47`+cos87`+···+cos327`=

详细步骤：由余弦的和差化积：cos α+cos β=2cos[(α+β)/2]·cos[(α-β)/2]得,cos7°+cos327°=2cos167°·cos160°cos47°+cos287°=2cos167°·cos120°cos87°+cos247°=2cos167°·cos80°cos127°+cos207°=...