#include
void main()
{
int i,sum1=0,sum2=0;
for(i=0;i<=10;i++)
{ sum1=sum1+i;
sum2=sum2+sum1;}
printf("%d\n",sum2);
}
an = 1+2+...+n
=n(n+1)/2
= (1/6)[ n(n+1)(n+2) -(n-1)n(n+1) ]
Sn= a1+a2+...+an
=(1/6)n(n+1)(n+2)
1+(1+2)+(1+2+3)+(1+2+3+4)+…...+(1+2+3+4+5+6+7+8+9+10)
=S10
=(1/6)(10)(11)(12)
=220
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