Question

Find the 20th term from the last term of the AP:3, 8, 13, ...., 253.​

Answer 1

AP : 3, 8, 13, ... , 253

first term, a = 3

common difference, d = 5

last term, l = 253

From the last, AP' = 253, 248, 243, 238, --- , 3

a = 253, d = -5, n = 20

a_n = a + (n - 1)d

= 253 + 19(-5)

= 253 - 95

= 158

Therefore, 20th term from last is 158.