Question

If the sum of the series is 513, then the number of terms are.

• A. 18
• B. 20
• C. 17
• D. None of these

Answer 1

Answer: (A)

18

Answer 2

According to condition

$\Rightarrow$ $513 = \frac { n } { 2 } \{ 2 \times 54 + ( n - 1 ) ( - 3 ) \}$

$\Rightarrow$ $1026 = n ( 111 - 3 n )$ $\Rightarrow$ $3 n ^ { 2 } - 111 n + 1026 = 0$

$\Rightarrow$ $( 3 n - 57 ) ( n - 18 ) = 0$

Hence $n = 18$ .