Question

find the sum of 20 terms of an ap 3, 7, 11 and so on

Answer 1

a20 = a + ( n- 1)× d
a20= 3 + [20 - 1]× 4
a20 = 3 + 76
a20= 79
sn = n÷2 (l + a)
sn = 20÷2 (79 + 3)

sn = 820