Notation. a; = a , + a2 + a3 + ... + an is a Greek letter sigma, this is an instruction to find the sum. i is the index of the sum, it tells where to start the sum and where to end it. Example: a1 = 1, a2 = 3, a3 = 5, a4 = 7, as = 9, a6 = 0, a7 = 2, ag = 4, ag = 6 Zi=1 X; is to add the terms xi, starting with i = 1, and ending with i = 5. a; = a1 + az + a3 + a4 + as = 1 + 3 + 5 + 7+9 = 1=1 Example: (Computer the following) 1. Zili = 1 + 2 + 3 +4+5 = 2. Ci= i2 = 12 + 22 + 32 + 42 + 52 = 3. Et=140 =