Some algebra with summation#
We use the symbol \(\Sigma\) for summation.
Say we have a series of four values \(x_1, x_2, x_3, x_4\).
We can write the sum \(x_1 + x_2 + x_3 + x_4\) as:
\[\Sigma_{i=1}^{4} x_i\]
You can read this summation as “the sum of values \(x\) subscript \(i\) from \(i=1\) through \(i=4\)”.
So:
\[
\Sigma_{i=1}^{4} x_i = x_1 + x_2 + x_3 + x_4
\]
When the indices of the summation are obvious, they may quietly disappear. For example, it may be obvious that we are summing over all \(i = 1, 2, 3, 4\), in which case we could write the \(Sigma\) with or without the indices:
\[\Sigma_{i=1}^{4} x_i = \Sigma x_i\]
Algebra of sums#
Addition inside sum#
Say we have two series of numbers \(x_1, x_2 cdots x_n\) and \(y_1, y_2 cdots y_n\).
\[\begin{split}\begin{aligned}
\Sigma_{i=1}^n (x_i + y_i) = \\
(x_1 + y_1) + (x_2 + y_2) + \cdots (x_n + y_n) = \\
(x_1 + x_2 + \cdots x_n) + (y_1 + y_2 + \cdots y_n) = \\
\Sigma_{i=1}^n x_i + \Sigma_{i=1}^n y_i
\end{aligned}\end{split}\]
Multiplying by constant inside sum#
\[\begin{split}\begin{aligned}
\Sigma c x_i = \\
c x_1 + c x_2 + \cdots c x_n = \\
c (x_1 + x_2 + \cdots x_n) = \\
c \Sigma x_i
\end{aligned}\end{split}\]
Sum of constant value#
\[
\Sigma_{i=1}^n c = n c
\]