in c language, what is the use of this formula after the colon:(n)\&(n-1)

In C language, the formula after the colon "(n)" is used in the syntax of a switch statement. It specifies the value or expression that the switch statement will compare each case label to.

For example, in the following switch statement:

switch(x){
  case 1:
    //code block
    break;
  case 2:
    //code block
    break;
  default:
    //code block
}

The formula "(x)" specifies the value or expression that the switch statement will compare each case label to. If `x` equals 1, the code block under the first `case` label will be executed. If `x` equals 2, the code block under the second `case` label will be executed. If `x` does not match any of the case labels, the code block under the `default` label will be executed.

Therefore, the formula after the colon in a switch statement is used to specify the value or expression to be compared with the case labels.