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JAMB UTME - Mathematics - 2006

Question 1 Report

How many terms of the series 3, -6, +12, - 24, + ..... are needed to make a total of 1-28?
Answer Details
3, -6, +12, -24
a = 3, r = -2
8 n = a ( 1 − r n ) 1 − r ∴ 1 − 2 8 = 3 ( 1 − ( − 2 n − 1 ) ) 1 − ( − 2 ) 1 − 2 8 = 3 ( 1 − ( − 2 n − 1 ) ) 3
1-28 = 1-(-2)n-1
-28 = -2n-1
8 = n-1
n = 9