Question

What is the common difference of an A.P. in which a21-a7=84

Answer 1

a21=a+20d
and a7 =a+6d

so,

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

14d=84

d=6

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Answer 2

a21= a+20d
a7 = a+6d
So, a21- a7 = 84
a + 20d - a-6d = 84
14d = 84
d = 84 /14 = 6