Question

What is common difference of an AP in which a21-a7=84?

Answer 1

a21-a7=84
a+20d-a+6d=84
a-a+20d+6d=84
26d=84
d=84÷26
d=42÷13

Answer 2

Answer:

a21-a7=84

a+(21-1)d - [a+(7-1)d]=84

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

a+20d-a-6d=84

14d = 84

d = 84/14

d = 6