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新版GRE数学复习重要考点：Sum of Arithmetic ProGREssion The sum of n-numbers of an arithmetic proGREssion is given by S=nxdn/2 where x is the first number and d is the constant increment. example: sum of first 10 positive odd numbers:101+2109/2=10+90=100 sum of first 10 multiples of 7 starting at 7： 107+7109/2=70+315=385 remember: For a descending AP the constant difference is negative. AP Average of n numbers of arithmetic proGREssion is the average of the smallest and the largest number of them. The average of m number can also be written as x + d/2. Example: The average of all integers from 1 to 5 is /2=3 The average of all odd numbers from 3 to 3135 is /2=1569 The average of all multiples of 7 from 14 to 126 is /2=70 remember: Make sure no number is missing in the middle. With more numbers, average of an ascending AP increases. With more numbers, average of a descending AP decreases. AP:numbers from sum given the sum s of m numbers of an AP with constant increment d, the numbers in the set can be calculated as follows: | 
