Find the sum of the first 20 terms of the sequences, -7, – 3, 1, . . . . .
-
620
- 660
- 690
- 1240
The correct answer is: A
Explanation
-7, - 3, 1, . .
T\(_1\) = -7, d = -3 - (-7) = -3 + 7 = 4
S\(_{20}\) = \(\frac{n}{2}[2a + (n - 1)d]
S\(_{20}\) = \(\frac{20}{2}[2(-7) + (20 - 1)4]
= 10[ -14 + 19(4)] = 10[62] = 620