编写一程序，求数列1,-3!,5!,-7!,…,(-1)n-1(2n-1)!前n项的和。

Question

不会填空啊.请帮忙
#include<stdio.h>
main()
{
int n,i,j,sign=1;
float fac,sum;
printf("please input value of n:");
scanf("%d",&n);
sum=0.0;
for(i=1;i<=2*n-1;i=i+2)
{
fac=1.0;
/*计算第n项*/
/*求前n项的和*/
sign=-sign;
}
printf("sum=%.0f\n",sum);
}

Answer 1

#include <stdio.h>

double fac(int n)
{
return n > 1 ? n * fac(n-1) : 1;
}

int main()
{
int n, i;
double total = 0;

printf("输入项数: ");
scanf("%d", &n);
for(i = 0; i < n; ++i)
{
if(i % 2 == 0)
total += fac(2 * i + 1);
else
total -= fac(2 * i + 1);
}

printf("前n项和(指数): %e\n", total);
printf("前n项和(double): %.lf\n", total);
}

Answer 2

/*计算第n项*/
/*求前n项的和*/
应为
for(j=1;j<=i;j++)fac*=j;//求第n项＝1*2*3*....
fac*=sign; //置正负
sum+=fac; //求n项和本回答被提问者采纳