Question

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

Answer 1

last term of the AP = 253
common difference= 8-3=5
20th term from the last term
= 253 - (20-1)(5)
=253-95
= 158

Answer 2

Given,
20th term from the last of the A.P..3,8,13....253
Now we have to find the reverse A.P.i.e..253...13,8,3
In the new A.P the 1st term a=253 and common difference  d=3-8=-5
We know,
⇒$a_{n} = a+(n-1)d$
⇒$a_{20} = 253+(20-1)(-5)$
⇒253 + 19 x(-5)
⇒253-95
⇒158