已知数列｛an｝为等比数列.⑴若a1 a2 a3=21,a1a2a3=216,求an

⑴若a1＋ a2＋ a3=21,a1a2a3=216,设a1=a2/q,a3=a2qa2/q＋a2＋a2q=21a2³=216=6³a2=66/q＋6＋6q=211/q＋q=5/2=1/2＋2所以q=2或1/21.q=2a1=6/2=3an=a1q的n-1次方=3×2的n-1次方2.q=1/2a1=6÷（1/2）=12an=a1q...