数列的片段和
n = int(input())
lst = list(map(float,input().split()))
num = 0
for i in range(n):
num += lst[i]*(i+1)*(n-i)
print('%.2f'%(num))
数列的片段和
n = int(input())
lst = list(map(float,input().split()))
num = 0
for i in range(n):
num += lst[i]*(i+1)*(n-i)
print('%.2f'%(num))