what is the common difference of an ap in which a21-a7=84

 Solution

Given that a21-a7=84

a+(21-1)d - { a+(7-1)d}=84

a+20d-(a+6d) =84

a+20d-a-6d=84

20d-6d=84

14d=84

d=84/14

d=6 Ans