Question

2. The number of terms of the series 24 +21 + 18+
such that the sum is 105 is
10
b. 11
c. Both a and b
d. None of these​

Answer 1

Step-by-step explanation:

a = t1 = 24

t2 = 21

d = t2 -t1 = 21-24 = -3

Sn = n/2 [2a +(n-1) d]

105 = n/2[48 -3n +3]

210 = 51n -3n^2

3n^2 - 51n +210 = 0

n = 10 or n = -7

So it is 10th term as no of terms cannot be -ve