已知数列{an}为等比数列.⑴若a1 a2 a3=21,a1a2a3=216,求an
问题描述:
已知数列{an}为等比数列.⑴若a1 a2 a3=21,a1a2a3=216,求an
⑵若a3a5=18,a4a8=72,求公比q.
答
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...