Question

Find the sum of the first 12 terms of the sequence. Show all work for full credit.

1, -4, -9, -14, . . .

Answer 1

The sum of 12 terms of AP = -318

Explanation:

Points to remember

Sum of n terms of an AP

Sₙ = n/2[ 2a + (n - 1)d]

Where n - number of terms

a - first term

d - common difference

To find the sum of 12 terms

The given AP is 1, -4, -9, -14, . . .

a = 1, d= -5 and n = 12

Sₙ = n/2[ 2a + (n - 1)d]

= 12/2[2*1 + (12 - 1)*(-5)]

= 6[2 + (11*(-5))]

= 6[2 - 55]

= 6 * (-53)

= -318

The sum of 12 terms of AP = -318