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

Posted by Sanam Singh 5 years, 4 months ago

CBSE > Class 10 > Mathematics

2 answers

Oav Ka Rajkumar ??? 5 years, 4 months ago

Given AP is 3,8,13........253 first term a=3 an=253 common difference =5 According to Question Let d=-5 a=253 an=3 n=20 a+(n-1)d =an =>253 +(n-1)-5 =3 =>(n-1)-5=-230 =>(n-1)=-250 /-5 =>n-1=50 =>n=50+1 =>n=51

0Thank You

Yogita Ingle 5 years, 4 months ago

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 , T₃₂= a+(32 -1)d
= 3 + 31 × 5 = 3 + 155 = 158

hence , 20th term from last = 158

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