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Math formulas: Arithmetic and geometric Series

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Notation:

Number of terms in the series: nn

First term: a1a_1

NthN^{th} term: ana_n

Sum of the first nn terms: SnS_n

Difference between successive terms: dd

Common ratio: qq

Sum to infinity: SS

Arithmetic Series Formulas:

an=a1+(n1)d a_n = a_1 + (n-1)d
ai=ai1+ai+12 a_i = \frac{a_{i-1} + a_{i+1}}{2}
Sn=a1+an2n S_n = \frac{a_1 + a_n}{2} \cdot n
Sn=2a1+(n1)d2n S_n = \frac{2 \cdot a_1 + (n-1) \cdot d}{2} \cdot n

Geometric Series Formulas:

an=a1qn1 a_n = a_1 \cdot q^{n-1}
ai=ai1ai+1 a_i = \sqrt{a_{i-1} \cdot a_{i+1}}
Sn=anqa1q1 S_n = \frac{a_nq - a_1}{q-1}
Sn=a1(qn1)q1 S_n = \frac{a_1 \cdot \left(q^n - 1 \right)}{q-1}
S=a11q,(for 1<q<1) S = \frac{a_1}{1-q}, \quad (\text{for } -1 < q < 1)

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