Part of ALG-03 — Sequences & Series (AP, GP, Special Series)

Sigma Notation and Manipulation

by Notetube Official98 words4 views

Properties of summation:

  • sum(c * aka_k) = c * sum(aka_k) (constant factor)
  • sum(aka_k + bkb_k) = sum(aka_k) + sum(bkb_k) (linearity)
  • sumk=1nsum_{k=1}^{n} c = nc (sum of a constant)
  • sumk=mnsum_{k=m}^{n} aka_k = sumk=1nsum_{k=1}^{n} aka_k - sumk=1m1sum_{k=1}^{m-1} aka_k (splitting range)

Decomposition technique: Express the general term as a sum of standard forms: tkt_k = Ak3Ak^3 + Bk2Bk^2 + Ck + D Then sum = Asum(k3k^3) + Bsum(k2k^2) + Csum(k) + Dn

Example: sum of k(k+1)(k+2) = sum(k3k^3 + 3k2k^2 + 2k) = [nn+12\frac{n+1}{2}]^2 + 3n(n+1)2n+16\frac{2n+1}{6} + 2nn+12\frac{n+1}{2}. But it's faster to use the direct formula n(n+1)(n+2)n+34\frac{n+3}{4}.

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