比如

        {   3x -5  ,1<= x <2

y =   {   2cosx + 1 , 2<= x <4

        {   sqrt(1+x^2)  , 4<= x <6

        {   x^2 -4x +5   , 其他

#include<iostream>
#include<math.h>
void main(){
double x , y;
cout <<"x = ";
cin >> x;
switch(int(x)){
   case 1:y =3 * x -5;break;
   case 2:
   case 3:y = 2 * cos(x) +1 ;break;
   case 4:
   csse 5:y = sqrt(1+x*X); brea;
   default: y = x*X - 4*x + 5;
}
   cout <<"y ="<<y<<endl;
}