what is the common defference of an ap in which a21-a7=84
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We know aₙ = a₁ + (n-1)d
Thus a₂₁ = a₁ + 20d
and a₇ = a₁ + 6d
Thus a₂₁ - a₇ = (a₁ + 20d) - (a₁ + 6d)
= 14d
According to question a₂₁ - a₇ = 84
∴ 14d = 84
or, d = 6
Thus the common difference is 6.
Thus a₂₁ = a₁ + 20d
and a₇ = a₁ + 6d
Thus a₂₁ - a₇ = (a₁ + 20d) - (a₁ + 6d)
= 14d
According to question a₂₁ - a₇ = 84
∴ 14d = 84
or, d = 6
Thus the common difference is 6.
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