#include <stdio.h>
#include <stdlib.h>
int main(){
    int x,y;
   
    printf("Input a number:");
    scanf("%d %d",x,y);
    if (x<0)
        {y=2*x+3;
        printf("%d",y);}
    else if((x>=0)&&(x<5)){
        y=2*x*x+3*x+4;
        printf("%d",y);}
    else if((x>=5)&&(x<10)){
        y=x*2+2*x+3;
        printf("%d",y);}
    else if(x>=10){
        y=x*2+5;
        printf("%d",y);}

    system("PAUSE");
    return 0;
}

scanf("%d %d",x,y);//scanf("%d%d",&x,&y);

楼主。。你 Input a number:然后输入两个数？

#include <stdio.h>

 #include <stdlib.h>

 int main(){
     int x,y;
     
     printf("Input a number:");
     scanf("%d",&x);
     if (x<0)
         {y=2*x+3;
         printf("%d",y);}
     else if((x>=0)&&(x<5)){
         y=2*x*x+3*x+4;
         printf("%d",y);}
     else if((x>=5)&&(x<10)){
         y=x*2+2*x+3;
         printf("%d",y);}
     else if(x>=10){
         y=x*2+5;
         printf("%d",y);}

     system("PAUSE");
     return 0;

 }