`d=5` और `S_10 =75` दिया है| a और ` a_10` ज्ञात कीजिए| एक `A.P.` में,

Explanation

`" " d=5`
तथा ` " " S_9 =75`
` rArr " " (9)/(2) [2a +(9-1) xx5] =75`
` rArr " " 2a +40 =(75xx2)/(9)`
` rArr " " a+20 =(75)/(9) " " rArr " " a+20 =(25)/(3)`
` rArr " " a=(25)/(3) -20 =(25-60)/(3)`
` rArr " " a=(-35 )/(3)`
तथा ` " "a_9 =a + 8d =(-35)/(3) +8xx5 =(-35)/(3) +40`
` " " =(-35+ 120)/(3) =(85)/(3)`
` therefore " " a= (-35)/(3) ` तथा ` a_9 =(85)/(3)`