新GRE数学重要考点1

>>GRE数学考点：新GRE数学重要考点1

>>GRE考试精品课程训练

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.

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