2006
Mathematics
67ca00ce0c643c71d77dee1f
How many terms of the series 3, -6, +12, - 24, + ..... are needed to make a total of 1-28?
A.
12
B.
10
C.
9
D.
8
Correct Answer: 9
Explanation
3, -6, +12, -24 a = 3, r = -2 8n=a(1−rn)/1−r. ∴1−28=3(1−(−2n-1))/1−(−2). 1−28=3(1−(−2n-1))/3 1-28 = 1-(-2)n-1 -228 = -2n-1 8 = n-1 n = 9
3, -6, +12, -24 a = 3, r = -2 8n=a(1−rn)/1−r. ∴1−28=3(1−(−2n-1))/1−(−2). 1−28=3(1−(−2n-1))/3 1-28 = 1-(-2)n-1 -228 = -2n-1 8 = n-1 n = 9THIS WEEK's
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