Question

nth term of a sequence is a+nb____

Answer 1

For a4 take a+3d and for a3 take a+2d
Now subtract 
a+3d-a+2d Equate this eq. 
So, +a and -a cancelled 
5d is equal to 0
So, the answer is d=1/5

Answer 2

Given sequence with n th term defined by tn= a + nb
So
tn-1= a + (n-1)b
⇒ tn- tn-1= a + nb - a - b(n-1)
⇒ tn- tn-1= b
Therefore, the difference between the preceding term and next term ofthis sequence is independent of 'n '

Hence this sequence is an AP with common difference b.