Using the formula for the sum of an arithmetic sequence, what is the sum of the first 20 terms?

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Using the formula for the sum of an arithmetic sequence, what is the sum of the first 20 terms?

2007-07-04 03:02:50 · 3 answers · asked by harmony 1 in Science & Mathematics ➔ Mathematics

3 answers

continuing ur previous question
sum of AP = n/2 [2a+(n-1)d]

sum of first 20 terms = 20/2[ 2(2) +(20-1)(2)]

=10 (4 +38)
=10 x 42
=420

2007-07-04 07:57:55 · answer #1 · answered by sweet n simple 5 · 0⤊ 0⤋

easiest way to do this is think of pairs of numbers
1-20 = 21
2-19=21
3-18=21
well we have 10 pairs of these so 10*21=210
The other way to do it is
n(n+1)/2

2007-07-04 10:21:58 · answer #2 · answered by leo 6 · 0⤊ 0⤋

Sn = (n/2).[ 2a + (n - 1).d ]
S20 = 10.[ 2a + 19 d ]
How are people finding values for a and d?

2007-07-08 10:09:22 · answer #3 · answered by Como 7 · 0⤊ 0⤋