Question

Find the sum of the first 20 terms of the AP: 5, 8, 11.....

Answer 1

a = 5

d = a2 -a1 = 8-5 = 3

sn = n/2(2a + (n-1)d)

s20 = 20/2(2(5) + (20-1)(3)]

s20 = 10(10+57)

s20 = 10(-67)

s20 = -670