Question

Find the 20th term from the last term 3,8,13_ _ _ _ _ _ _ _ _ 253 .

Answer 1

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  $Tn=a+(n-1)d$

=> 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 ,  $T_{32} =a+(32-1)d$

= 3 + 31 × 5 = 3 + 155 = 158  

hence , 20th term from last = 158

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