Question

Find the 20th term of the ap, of the ap is:3,8,13.....253

Answer 1

$\bf\large\underline\pink{Solution:-}$

Given series is 3, 8, 13, ...... 253

here, first term , a = 3

common difference , d = 5

Let us find total number of terms at first.

use

=> 253 = 3 + (n - 1)5

=> 250 = 5(n - 1)

=> n - 1 = 50

=> n = 51

so, there are 51 terms in given series.

now we know, mth term from last =last term - mth term + 1

so, 20th term from last = 51 - 20 + 1 = 32

hence, 20th from last = 32th term from first

use ,

= 3 + 31 × 5 = 3 + 155 = 158

hence , 20th term from last = 158

Answer 2

a = 3, t1 = 3, t2 = 8

t1 - t2 = 8 - 3

= 5

d = 5

tn = a + (n-1) d

t20 = 3 + (20-1) 5

= 3 + 19 x 5

= 3 + 95

= 98

Answer : The 20th term of this A.P. is 98