A good answer might be:

Inspect the documentation. What is the type of the argument expected by sqrt()?

double

What is the type of value returned by sqrt()?

double

Is sqrt() a static method?

Yes

NaN

Looking further at Sun's documentation for sqrt() we find details:

Returns the correctly rounded positive square root of a double value. If the argument is NaN or less than zero, the result is NaN.

NaN stands for "Not a Number". This is a 64-bit pattern that is returned by sqrt() when its argument is not correct. Here is an example run of the program: 

C:\chap11>java SquareRoot
Enter a double: -3
square root   : NaN

Here the method returned the 64-bit pattern NaN. The println() method writes the characters "NaN" when it sees this pattern. (The actual bit pattern is not character data.)

In a highly secure, industrial-strength programming language like Java, the behavior of a function must be described for all possible input, both in range and out of range. The documentation describes all of these cases. But you don't need to worry about this now. If your program writes out "NaN" but you gave it valid data, you probably have a logic error.

QUESTION 11:

What do you expect is the output of the following program fragment: 

int x = 9;
System.out.println( Math.sqrt( x ) );