Question

The first term of an A. P is -18 and last term is 414. If the common difference is 8 . Find the sum of all terms of this A.P.

Answer 1

Answer:

sn = 10890

Step-by-step explanation:

a = -18

an = 414

d = 8

n = ?

sn = ?

Now,

an = a + (n-1)d

414 = -18 + (n-1) × 8

414 = -18 + 8n-8

414+18 = 8n-8

432 = 8n-8

432+8 = 8n

440 = 8n

n = 440/8

n = 55

Now,

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

or

n/2 [a + an] l is the last term.

Now,

sn = 55/2 [-18 + 414]

sn = 55/2 × 396

sn = 55 × 198

sn = 10890

Hope it helps..........