` a=7 ` और `a_13 =35` दिया है| d और `S_13` ज्ञात कीजिए| एक `A.P.` में

Explanation

` " " a=7`
तथा ` " "a_13 =35 " " rArr " " a+ (13-1) d=35`
` rArr " ' 7+ 12d =35 " " rArr " " 12d =35 -7 =28`
` " " d = (28)/(12)=(7)/(3)`
तथा ` " " S_n =(n)/(2) [2a +(n-1)d]` से
`rArr " " S_13 =(13)/(2) [2xx7+(13-1)xx(7)/(3)]`
` " " =(13)/(2) [14+28 ] =(13)/(2) xx42 =273`
` therefore " " d =(7)/(3) ` और ` S_13 =273`