[编程题]等差数列
等差数列
http://www.nowcoder.com/questionTerminal/f792cb014ed0474fb8f53389e7d9c07f
数学思路，转化为等差数列求和，直接用公式计算求解，而不是循环遍历，附python代码
import sys
for s in sys.stdin:
try:
n = int(s)
d = 3
a1 = 2
an = a1 + (n-1)*d
sums = (a1 + an)/2 * n
print(int(sums))
except:
print(-1)
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