Question

Find the sum given for each of the following series:(a) Sn=1+5+9+....(n=15)
(b) Sn=(-8)+(-3)+2+... (n=20)

Answer 1

Answer:

(i) Sn = 285

(ii) Sn = 790

Step-by-step explanation:

Sn = n/2 [2a + (n - 1)d ]

(i) a = 1,

d = 5 - 1 = 4

n = 15

Sn = 15/2 [ 2(1) + (15 - 1)4 ]

= 15/2 ( 2 + 56 )

= 15 × 19 = 285

(ii) a = (-8)

d = (-3) - (-8) = 5

n = 20

Sn = 20/2 [ 2 (-8) + (20 - 1)5 ]

= 10 [ (-16) + 95 ]

= 10 × 79 = 790