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Unit 2 Tutorials
Question 20
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For the arithmetic sequence beginning with the terms (-2,0,2,4,6,8...), What is the sum of the first
18 terms?
0 238
0 340
o 304
0 270
Sove and continue

Answer 1

270

Explanation:

For any arithmetic sequence

nth term is given by

nth term = a + (n-1)d

where a is first term,

d is common difference

d is given by nth term - (n-1)th term

sum of n terms given by

sum = n/2(2a + (n-1)d)

________________________________________________

Given arithmetic sequence

-2,0,2,4,6,8...

first term a = -2

lets take third term as nth term and second term as (n-1)th term to find common difference d.

d = 2 - 0 = 2

using a = -2 , d = 2, n = 18

thus, sum of first 18 terms = n/2(2a + (n-1)d)

=18/2( 2*(-2) + (18-1) 2)

=9 ( -4 + 34)

=9 ( 30) = 270

Thus, sum of first 18 terms is 270.