Question

In an AP, the $4th$ term is $36$. The $21st$ term is $108$ more than the $9th$ term. Find the common difference

• A. $12$
• B. $9$
• C. $4$
• D. $-3$

Answer 1

The correct option is B $9$

Arithmetic Progression

Given $T_4=36$

$T_{21}=108+T_9$

$d=?$

General term of an A.P

$T_n=a+(n-1)d$

$T_{21}=108+T_9$

$a+(21-1)d=108+a+(9-1)d$

$20d=108+8d$

$12d=108$

$d=9$