Question

15
Find the 20th term from the last term of the A.P 3,8,13.........253.
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Answer 1

Answer:

First, we will find the number of terms in the sequence.

t

n

=a+(n−1)d

⟹253=3+(n−1)(5)

⟹253=5n−2orn=51.

So, there are a total of 51 terms in the sequence.

The 20th term from the last is same as the 32nd term from the first.

Hence, t

32

=3+(32−1)(5)

⟹t

32

=3+31(5)

⟹t

32

=158..

Answer 2

a=3,d=5

253=3+5(n−1)

Or, 5(n−1)=250

Or, n−1=50

Or, n=51

So, the 20th term from the last term =51−19=32nd

term

Now, n32=3+5×31=158