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The first term of an Arithmetic progression is 3 and the ...

The first term of an Arithmetic progression is 3 and the fifth term is 9. Find the number of terms in the progression if the sum is 81

A.

12

B.

27

C.

9

D.

4

E.

36

Correct answer is C

1st term a = 3, 5th term = 9, sum of n = 81

nth term = a + (n - 1)d, 5th term a + (5 - 1)d = 9

3 + 4d = 9

4d = 9 - 3

d = 64

= 32

= 6

Sn = n2(6 + 34n - 32)

81 = 12n+3n24 - 3n

= 3n2+9n4

3n2 + 9n = 324

3n2 + 9n - 324 = 0

By almighty formula positive no. n = 9
= 3