WAEC - Further Mathematics (2019 - No. 12)
Find the sum of the first 20 terms of the sequence -7-3, 1, ......
620
660
690
1240
Selitys
d = -3 - (-7) = 4
S\(_{20}\) = \(\frac{20}{2}\){2(-7) + (20 - 1) 4}
= 10(- 14 + 76)
= 620
d = -3 - (-7) = 4
S\(_{20}\) = \(\frac{20}{2}\){2(-7) + (20 - 1) 4}
= 10(- 14 + 76)
= 620