Question

Two APs have the same common difference. The first term of one of these is $-1$ and that of the other is $-8$. Then the difference between their $4th$ terms is

• A. $-1$
• B. $-8$
• C. $7$
• D. $-9$

Answer 1

The correct option is C $7$

Let the same common difference of two A.P. be $d$
First term of 1st AP $=-1$
Forth term of 1st AP $=(-1+3d)$
First term of 2nd AP $=-8$
Forth term of 2nd AP $=(-8+3d)$
Then $(a_4{)}_{1stAP}-(a_4{)}_{2ndAP}=(-1+3d)-(-8+3d)$
$=-1+3d+8-3d=7$