If p, q, r are in increasing AP, then common difference will be

$(a)\;\sqrt{p^2-qr}\qquad(b)\;\sqrt{q^2-pr}\qquad(c)\;\sqrt{r^2-pq}\qquad(d)\;None\;of\;these$

Answer : (b) $\;\sqrt{q^2-pr}$
Explanation : Let $\;p=a-d$
$q=a$
$r=a+d$
$p\;r=a^2-d^2$
$q^2=a^2$
$d^2=q^2-pr$
$d=\sqrt{q^2-pr}\;.$