Question

if 8th term of an a.p is equal to 5 times its second term if its 5th term is 18 find the sum of the first 10 terms

Answer 1

8th term = 5 times of 2nd term

a+7d = 5(a+d)
a + 7d = 5a + 5d

4a = 2d

d = 2a

Also given
5th term = 18
a + 4d = 18
a + 8a = 18   (Substituting d = 2a)
9a = 18
a = 2
So, d = 4

The AP is
2, 6, 10, 14, 18, 22, 26, 30, 34, 38, 42, 46, 50