Question

The sum of n natural numbers is 5n²+4n.Answer the following. i)The common difference is ii) the 8th term is? iii) sum if 1st 8th term iv) which term if the series is 119?​

Answer 1

Given: Sn = 5n2 - 4n

straight t subscript 8 equals straight S subscript 8 minus straight S subscript 7

equals open parentheses 5 cross times 8 squared minus 4 cross times 8 close parentheses minus open parentheses 5 cross times 7 squared minus 4 cross times 7 close parentheses

equals 5 cross times open parentheses 8 squared minus 7 squared close parentheses minus 4 cross times open parentheses 8 minus 7 close parentheses

equals 5 cross times 15 minus 4

equals 75 minus 4

equals 71

Hence, the 8th term is 71.