Question

If first and last term of an ap is 4 and 81 and common differesnce is 7. Then find no. Of terms and sum of that ap

Answer 1

Answer:n = 12

S12 = 510

Step-by-step explanation:

a = 4

d = 7

l = 81

a + (n-1) d = 81

4 + (n-1) 7 = 81

4 + 7n -7 = 81

7n = 84

n = 12

S12 = n/2 ( a + l )

=12/2 (4 + 81)

=6 X 85

=510