Question

请用c语言计算： S=1*2-2*3+3*5-5*8+8*13-13*21+……（前20项的和）？

Answer 1

#include <stdio.h>

int main() {

  int a = 1, b = 2, sum = 0;

  for (int i = 1; i <= 20 / 2; i++) {

    sum += a * b - b * (a + b);

    a = a + b;

    b = a + b;

  }

  printf("%d\n", sum);

  return 0;

}

Answer 2

据观察，与 斐波那契数列 有关

 int i=1,t=0,a,b,s=1;
long r;
while(i<=20)
{ if (i<3)
f[i]=1;
else
f[i]=f[i-2]+f[i-1];
if (i==1)
a=1;
else
a=b;
b = a + f[i];
t = s*a*b;
r += t;
printf("%3d %8d %8d %8d %10d\n",i,s,a,b,r);
s = -s;
i++;
}

Answer 3

#include<stdio.h>

#include<math.h>

int main()

{

long a=1,b=1,t=1,sum=0;

for(int i=1;i<=20;i++)

{

if(i%2==0)

a=-a;

b=b+abs(t);

sum+=a*b;

printf("a= %-10d b= %-10d sum= %-10d\n",a,b,sum);//a,b,sum的变化情况 

t=a;

a=b;

}

printf("前20项的和是：%ld",sum);

return 0;

}