Question

1) Find the sum to 18 terms of the following serie
- 12-9-6...​

Answer 1

Hello !!

Is very simple solve your question. See the resolution below.

Find the common difference.

d = (An - Ak)/(n - k)

d = (-9 - (-12))/(2 - 1)

d = (-9 + 12)/(2 - 1)

d = 3/1

d = 3

Now, you find the sum of the first 18 terms.

Sn = (n/2) × [2Ak + (n - k) × d]

S18 = (18/2) × [2(-12) + (18 - 1) × 3]

S18 = 9 × [-24 + 17 × 3]

S18 = 9 × [-24 + 51]

S18 = 9 × 27

S18 = 243

Final result : 243 is the sum of the first 18 terms.

I hope I have collaborated !