Let's try going the other way around. {{CMD_56}} Write the following sum in sigma notation. {{MATH_57}} {{CMD_58}} Notice that we can factor a {{MATH_59}} out of each term to rewrite this sum as {{MATH_60}} That means that we are adding together {{MATH_61}} times every number between {{MATH_62}} and {{MATH_63}}. If we use {{MATH_64}} as our index, the sigma notation could be {{MATH_65}} There is no need to use {{MATH_66}} as our index variable. We could have just as easily used {{MATH_67}} or {{MATH_68}} instead. {{MATH_69}} Notice, that these are NOT the same as {{MATH_70}} {{CMD_71}} {{CMD_72}}