Не работает switch - C (СИ) (79068)

// 123.cpp : Defines the entry point for the console application.
//
#include <StdAfx.h>
#include <stdio.h>
#include <math.h>
#include <stdlib.h>
 
void zadanie1 ()    {
 
    int x,y;
    printf("vvedite x,y:");
    scanf("%d%d",&x,&y);
    if  ((x>0) && (y>0))
    printf("pervaya chetvert");
    if  ((x<0) && (y>0))
    printf("vtoray chetvert");
    if  ((x<0) && (y<0))
    printf("tretaya chetvert ");
    if  ((x>0) && (y<0))
    printf("chetvertaya chetvert ");
    if  ((x=0) && (y=0))
    printf("nachalo koordinat");
    if (x=0) printf("Error");
    if (y=0) printf("Error");
    system("pause");
    
}
void zadanie2 ()
{
    int a,b,c;
    double d,x1,x2;
    printf("vvedite peremennie a,b,c:");
    scanf("%d%d%d",&a,&b,&c);
    d=b*b- 4 *a*c;
    if (d<0) printf("Error");
    else
    {   d=sqrt(d);
    x1= (-b+sqrt(d))/(2*a);
    x2= (-b-sqrt(d))/(2*a);
    
    if (x1==x2)printf("x1=%f\n",x1);
    else
        printf("x1=%f\nx2=%f\n",x1,x2);}
        system("pause");
 
}
 
void zadanie3 ()
{
    int i,n,s;
    printf("vvedite chislo n factorial:");
    scanf("%d",&n);
    for (s=1,i=1;i<=n;i++);
        s=s*i;
    printf("s=%d\n",s);
    system ("pause");
 
}
 
void zadanie4 ()
{
    int n,a,i,s;
    printf("vvedite chislo fibonachi n:");
    scanf("%d",&n);
    for (a=1,i=0;i<=n;a=i,i++);
    s=a+i;
    printf("s=%d",s);
    system ("pause");
    return;
}
 
void zadanie5 ()
{int  i,x;
double y;
printf ("vvedite znachenie peremennoi x :");
scanf ( "%d",&x);
for (y=0,i=1; i<20; ++i);
y=i*x+i+y;
printf("y=%f",y);
system ("pause");
 
}
void zadanie6 ()
{
    int i,x,n;
    double y;
        printf("vvedite znacheniya peremennih x,n:");
        scanf("%d%d",&x,&n);
    for (y=n/(x+1),i=1;i<n;i++);
    y=y+(n-i)/(x+i+1);
        printf("y=%f",y);
    system ("pause");
}
 
void zadanie7 ()
{
 
    int x,x1,x2,s,y,min,max,m1,m2;
    printf("enter a segment [x1,x2]:");
    scanf("%d%d",&x1,&x2);
    printf("vvedite shag:");
    scanf("%d",&s);
    m1=x1;
    m2=x2;
    if (x1>0)
    {min=x1*x1*x1-9*x1;
    max=x1*x1*x1-9*x1;}
    else
    {min=x1*x1*x1*x1+2*x1*x1*x1-9*x1*x1-3*x1+6;
     max=x1*x1*x1*x1+2*x1*x1*x1-9*x1*x1-3*x1+6;}
    x=x1;
    if (x>0)  y=x*x*x-9*x;
    else y=x*x*x*x+2*x*x*x-9*x*x-3*x+6;
 
    if (y>max)
    {max=y;
    m2=x;}
 
    printf("max=%d,x=%d)\n",max,x);
    printf("min=%d,m1=%d)\n",min,m1);
    system("pause");
 
}
void print_menu() {
    printf("1. kvadrant\n");
    printf("2. vichislenie kornei uravneniya\n");
    printf("3. vichislenie faktoriala\n");
    printf("4. vichislenie chisel fibonachi\n");
    printf("5. vichislenie znachenie funkcii 1\n");
    printf("6. vichislenie znachenie funkcii 2\n");
    printf("7. znachenie funkcii na otrezke\n");
    printf("0. exit\n");
}
 
int main() {
    int c;
    print_menu();
    printf("vvedite nomer\n");
    scanf("%d", &c);
    switch(c) {
    case 1:zadanie1; break;
    case 2:zadanie2; break;
    case 3:zadanie3; break;
    case 4:zadanie4; break;
    case 5:zadanie5; break;
    case 6:zadanie6; break;
    case 7:zadanie7; break;
        }
    system ("pause");
    return 0;
 
    }