# What is the

Let the common difference be 'd' and the first term be 'a'.

And we know that, nth term of an AP is given by

an = a + (n - 1) d

where n is the nth term
a is the first term
d is the common difference

Given:  a21 - a7 = 84

a21 = a + (21 - 1) d                                  .........eq(1)

a= a + (7 - 1) d                             .................eq(2)

Subtracting eq (2) from eq(1):

a21 - a7 = a + 20d - (a + 6d) = 84

⇒ 14d = 84

⇒ d = 6

Hence, the common difference of AP is 6.

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