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
12
27
9
4
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
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