Question

itong

求救《急》

102
8² = 18x-4=-27. Y=-3.
-243=(-3)5
5. 若一個等比數列的首項為6,第三項比第二項多120,則此數列的前四項為何?
n+1=5
答:

Answer 1

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綾

a1 = 6
a3 - a2
= a1*r^2 - a1r
= 6(r^2 - r)
= 120

r^2 - r - 20 = 0
(r - 5)(r + 4) = 0
r = 5 or -4

r = 5
a1 = 6
a2 = 30
a3 = 150
a4 = 750

r = -4
a1 = 6
a2 = -24
a3 = 96
a4 = -384

itong

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