Question

an ap 5,12,19.... has 50 term find its last term hence find the sum of first 20 terms of an a.p ​

Answer 1

check the attachment

Answer 2

Answer:

Solution :-

a = 5

d = 12 - 5 = 7

n = 50

an = a + (n - 1)d

an = 5 + (50 - 1)7

an = 5 + 49*7

an = 5 + 343

an = 348

Sum of the last 25 terms -

a = 348, d = - 7, n = 15

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

Sn = 15/2[2*348 + (15- 1) - 7]

Sn = 7.5[696 + (14 × - 7)]

Sn = 7.5[696 - 98]

Sn =  7.5*598

Sn = 4485

Sum of the last 15 term is 4485