等比数列:2,6,18,54~的第50项是( ),前50项的和是( ).
问题描述:
等比数列:2,6,18,54~的第50项是( ),前50项的和是( ).
答
a1 = 2
q = 3
a50 = 2 * 3^49
S50
= 2 * (1 - 3^50) / (1 - 3)
= 3^50 - 1