The seconds term of a geometric series is 4 while the fourth term is 16. Find the sum of the first five terms

Assessment: JAMB UTME - Mathematics - 2011 Subject: General Mathematics

Question 1 Report

60

62

54

64

Answer Details

T2 = 4, T4 = 16
Tx = arn-1
T2 = ar2-1 = 4 i.e. ar3 = 16, i.e. ar = 4
T4 = ar4-1
therefore, $\frac{T_{4}}{T_{r}}$ = $\frac{a r^{3}}{a r}$ = $\frac{16}{4}$
r2 = 4 and r = 2
but ar = 4
a = $\frac{4}{r}$ = $\frac{4}{2}$
a = 2
Sn = $\frac{a (r^{n} - 1)}{r - 1}$
S5 = $\frac{2 (2^{5} - 1)}{2 - 1}$
= $\frac{2 (32 - 1)}{2 - 1}$
= 2(31)
= 62

Read lesson note on Sequence And Series (WAEC)

Sequence And Series

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