Question

how many terms of Ap:-15,-13,-11...... must be taken to give a sum of -55? explain reason for double answer.

Answer 1

AP = -15,-13,-11,....

Here,

a = -15

d = 2

Sn = -55

Sn = n/2[2a+( n-1)×d]

PUT VALUES AND SOLVE.U WILL GET THE NTH TERM

Answer 2

Hola Friend ✋✋✋

S = - 55

a = - 15

d = 2

n = ?

$s_{n} = \frac{n}{2} \times (2a + (n - 1)d) \\ \\ - 55 = \frac{n}{2} \times ( - 30 + (n - 1) \times 2) \\ \\ - 55 = n \times ( - 15 + n - 1) \\ \\ - 55 = - 16n + {n}^{2} \\ \\ {n}^{2} - 16n + 55 = 0 \\ \\ {n}^{2} - 5n - 11n + 55 = 0 \\ \\ n(n - 5) - 11(n - 5) = 0 \\ \\ (n - 11)(n - 5) = 0 \\ \\ so... \\ \\ n = 11 \: or \: 5$


Hope it helps