8.17

#include "stdio.h"

void fun(double *x,double *y){
 *x=*x+*y;
 *y=*x-*y*2;
}

void main(){
 double m,n;
 double *x,*y;
 x=&m;
 y=&n;
 scanf("%lf%lf",x,y);
 fun(x,y);
 printf("%lf,%lf",*x,*y);
}

8.18

#include "stdio.h"

double fun(double *x,double *y,double *z){
 double man1,min1;
 double *max=&man1,*min=&min1;
 if (*x>*y) {
  if (*y>*z) {
     *max=*x;
     *min=*z;
  }
  else {
    if(*y>*z) {
       *max=*y;
       if(*x>*z) *min=*z;
       else *min=*x;
    }
    else{
      *max=*z;*min=*x;
    }
  }
 }
}

void main(){
 double m,n,l;
 double *x,*y,*z;
 x=&m;
 y=&n;
 z=&l;
 scanf("%lf%lf",x,y,z);
 fun(x,y,z);
 printf("%lf,%lf",*max,*min);
}