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Answer 1 · 2018-05-10T19:23:05

Tn = #3*2^(n-1)#

3, 6, 12, 24....

a = 3 First term
r = #6/3# = 2
n = ?

Tn = #a.r^(n-1)# = #3*2^(n-1)#

Answer 2 · 2018-05-10T19:29:59

The fourth selection #t_n = 3(2)^(n-1)#

Given:

#3,6,12,24,...#

Starting with #n = 1#

The first selection:
#t_1 = -2(3)^(1-1) = -2 larr# does not match so we reject it

The second selection:

#t_1 = 2(3)^(n-1) = 2 larr# does not match so we reject it

The third selection:

#t_1 = -3(2)^(1-1) larr# does not match so we reject it

The fourth selection:

#t_1 = 3(2)^(1-1) = 3#
#t_2 = 3(2)^(2-1) = 6#
#t_3 = 3(2)^(3-1) = 12#
#t_4 = 3(2)^(4-1) = 24#

All 4 terms match.