The sum of the first 50 positive even integers is 2,550. What is the sum of the even integers from 102 to 200, inclusive?
5,100
7,550
10,100
15,500
20,100
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102至200视为等差数列，An=200, A1=102, d=2, 求出n=50.
102至200每个数都比（前50个偶数）中的每个数多100，所以50*100
答案：2550+50*100=7550
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- > Since A.P [ (200+102)/2 ] = 151 = Average
- > Avg = Sum / n
- > Sum = 151 * 50 = 7550
if A、B、C， positive integers，B=（A+C)/2
so Sum (A+B+C)=B*3
- > Since A.P [ (200+102)/2 ] = 151 = Average
- > Avg = Sum / n
- > Sum = 151 * 50 = 7550
if A、B、C， positive integers，B=（A+C)/2
so Sum (A+B+C)=B*3