pls explain ans on
http://www.merit nation.com/cbse/class12-sc ience/textbook solutions/physics/hc-ver ma---ii/electromagnetic-ind uction/page304 -obje ctive-i-q no1/14 03_6941_7 4996/

SPECIFIC DOUBT: de=bvl=bwxdx in this wx will represent v that i agree but where did from dx come what do it represent , not l obviously as it is the small length of the small element , shud not there be x instead of dx in ans