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Question
The nth term of a G.P. is 3.(–2)n. Find the sum of its 7 terms.
Sum
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Solution
N = 1, a1 = 3.(–2)1
= –6
N = 2, a2 = 3.(–2)2
= 3.4
= 12
r = `12/(-6)`
= –2
`S_n = (a(r^n - 1))/(r - 1)`
`S_7 = (-6.((-2)^7 - 1))/(- 2 - 1)`
`S_7 = (-6.(-128 - 1))/(- 3)`
S7 = 2.(−129)
S7 = −258
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