Question

Sum of 5th and 7th term of A.P is 30 . If its 25th term is 3 times its eighth term find A.P

Answer 1

A5 + A7 =30
A +4d + a + 6d =30
2a + 10d =30
2a = 30 - 10d
a= 30/2- 10d/2
a= 15- 5d. --------(1)

a25 = 3×a8
a + 24d = 3× (a+ 7d)
a + 24d = 3a + 21d
3a -a = 24d - 21d
2a= 3d
2(15-5d) =3d
30 - 10d = 3d
30= 3d + 10d
30= 13d
d= 30/13