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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=nx*dn(n-1)/2

　　where x is the first number and d is the constant increment.

　　example:

　　sum of first 10 positive odd numbers:10*1+2*10*9/2=10+90=100

　　sum of first 10 multiples of 7 starting at 7: 10*7+7*10*9/2=70+315=385

　　remember:

　　For a descending AP the constant difference is negative.