作业 补 第六题真心不会

#include<stdio.h>
int main()
{
    int i;
    double ave,sum;
    int a[4];
    printf("Enter 4 integers:");
    sum=0;
    for(i=0;i<4;i++){
        scanf("%d",&a[i]);
        sum=sum+a[i];
    }
    ave=sum/4;
    printf("ave=%.1f
",ave);
    printf("sum=%.0f
",sum);
    return 0;
}

#include<stdio.h>
#include<math.h>
int main()
{
    double x,cost;
    printf("Enter x:");
    scanf("%lf",&x);
    if(x>0&&x<=50)
        printf("cost=%.2f
",cost=0.53*x);
    else if(x>50)
        printf("cost=%.2f
",cost=50*0.53+(x-50)*0.58);
    else
        printf("Invalid value!");
        return 0;
}

#include<stdio.h>
int main()
{
    int i,n,m,sum1,sum2;
    int denominator;
    double item,sum;
    printf("Enter m:");
    scanf("%d",&m);
    printf("Enter n:");
    scanf("%d",&n);
    sum=sum1=sum2=0;
    for(i=m;i<=n;i++){
        sum1=sum1+m*m;    
    }
    for(i=m;i<=n;i++){
        item=1.0/denominator;
        sum2=sum2+item;
        denominator=denominator+1;
    }
    sum=sum1+sum2;
    printf("Sum of number from %d to %d is %.6lf
",m,n,sum);
    return 0;
}

#include<stdio.h>
int main()
{
    int denominator,flag,i,n;
    double item,sum;
    printf("Enter n:");
    scanf("%d",&n);
    flag=1;
    denominator=1;
    sum=0;
    for(i=1;i<=n;i++){
        item=flag*i*1.0/denominator;
        sum=sum+item;
        flag=-flag;
        denominator=denominator+2;
    }
    printf("sum=%f
",sum);
    return 0;
}

#include<stdio.h>
#include<math.h> 
int main ()
{
    int i,n;
    double y,sum;
    sum=0;
    printf ("Enter n:");
    scanf("%d",&n);
    for(i=1;i<=n;i++){
        y=sqrt(i);
        sum=sum+y;    
    }
    printf("sum=%.2lf
",sum);
    return 0;
}