Question

Question 17
Find the ${20}^{th}$ term from the last term of the A.P. 3, 8, 13, …, 253.

Answer 1

Given A.P. is:
3, 8, 13, …, 253
Common difference of this A.P. is 5.
Therefore, this A.P. can be written in reverse order as;
253, 248, 243, …, 13, 8, 5
For this A.P,
a = 253
d = 248 − 253 = −5
n = 20
$a_{20}=a+(20-1)d{a}_{20}=253+\left(19\right)(-5)a_{20}=253-95a=158$
Therefore, ${20}^{th}$ term from the last term is 158.