Two APs have the same common difference.

(c) Let the common difference of two APs are d1 and d2, respectively.

Bycondition,                          d1 =d2 =d                                                                      …(i)

Let the first term of first AP (a1) = -1

and the first term of second AP (a2) = – 8

We know that, the nth term of an AP, T1 = a + (n – 1) d

∴4th term of first AP, T4 = a, + (4 – 1)d = – 1 + 3d .

and 4th term of second AP, T’4 = a2 + (4 – 1)d = – 8 + 3d

Now, the difference between their 4th terms is i.e.,

|T4 -T’4|= (-1 + 3d)-(-8+3d)

= -1+ 3d + 8 – 3d = 7

Hence, the required difference is 7.