The sigma symbolsignifies summation.
means add up all the numbersThere are several different, similar variations of notation, so thatandmean essentially the same thing, though the second example means explicitly 'add up all thefromto'.
Summation in this way has several useful properties:
Summation is also linear
This last property can be useful. For the second example above withwe foundWe could have used the linearity property to write
There are standard expressions forand