Question

Find the sum of the arithmetic sequence.

-1, 2, 5, 8, 11, 14, 17

Answer 1

If your just adding and finding the sum it would be 58

Explanation:

1+2+5=8

8+8=16

16+11=27

27+14=41

41+17=58

Answer 2

S₇ = 16

Explanation:

First term = -1

common difference d = 3 ; n = 7

The sum of first n terms Sₙ = 2 a + (n - 1) d

S₇ = 2 × -1 + ( 7 -1 ) 3

= - 2 +( 6 × 3)

= - 2 + 18

= 16