learn about sigma notation and arithmetic summations

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+[Σ Sigma notation](https://www.khanacademy.org/math/ap-calculus-ab/ab-integration-new/ab-6-3/v/sigma-notation-sum)
+Sum of some terms
+Sum of the first 10 #'s
+
+```
+-------
+ 10   |                                        |
+ Σ  i | 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10 | 55
+ i=1  |                                        |
+-------
+
+-------
+ 100  |
+ Σ  i | 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10 .... 100
+ i=1  |
+-------
+
+sum := 0
+for i = 1; i <= 100; i++ {
+  sum += i
+}
+
+```
+
+The summation is an arithmetic series and has the value
+```
+-------
+ n    |                  |
+ Σ  k | 1 + 2 + ... + n  |
+ k=1  |                  |
+-------
+```
+
+Sum of squares:
+```
+-------
+ n      |    n(n+1)(2n+1)
+ Σ  k^2 | = --------------
+ k=0    |         6
+-------
+
+-------
+ n      |    n^2(n+1)^2
+ Σ  k^3 | = -------------
+ k=0    |        4
+-------
+```