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Sequences and Series

General Forms of Arithmetic Progression (A.P)

If the first term $=a$ and the common difference $=d$.

General form of an AP is $a, a+d, a+2 d, a+3 d,$ ........

If three or four numbers in AP are to be considered, they can be taken conveniently as

$a-d, a, a+d$ or $a-3 d, a-d, a+d, a+3 d$.

General Forms of Geometric Progression (G.P)

If the first term $=a$ and the common ratio $=r$.

- General form of a GP is $a, a r, a r^2, a r^3, \ldots$.

- If three or four numbers in GP are to be considered, they can be taken conveniently as

$$ \frac{a}{r}, a, a r \text { or } \frac{a}{r^3}, \frac{a}{r}, a r, a r^3 \text {. } $$