find the sum of the sequence -11,-16,-21,-26,...a15(Show your solution)
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find the sum of the sequence -11,-16,-21,-26,...a15(Show your solution)
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PROBLEM
Find the sum of the first 15 terms of the arithmetic sequence 9, 12, 15,...
Given: a1 =_____ ; d =____ ; n = ____
ANSWER
Given:
a1 = 9
d = 3
n = 15
The sum of the first 15 terms is 450
Solution
Getting the sum of the first 15 terms
s15 = n/2 [ 2a1 + (n-1)(d) ]
s15 = 15/2 [ 2(9) + (15-1)(3) ]
s15 = 15/2 [ 18 + (14)(3) ]
s15 = 15/2 [ 18 + 42 ]
s15 = 15/2 (60)
s15 = 900/2
s15 = 450
Answer:
-74 I think that's the answer