JavaScript functions and global variables
Yes, you are right, decimal, a, b and c are not accessible by other functions because they are local to getValue(). To make them global, you need to declare them outside of your function, like this: 

Solution #1

var decimal; 
var a;
var b;
var c;

function getValue(){
 decimal = // assign decimal
 a = // assign a
 // assign rest of the variables
}

function useVariables(){ 
   return a + b + c;
}


Using global variables, however, is considered a bad practice so I would move onto another solution. 

Solution #2

Have 'getter functions' for each of your variables. Instead of having getValue(), have getDecimal(), getA(), getB() and getC() functions (you might consider giving these variables more meaningful names). Your code would turn into something like this: 

function getA(){
  return parseFloat(parent.document.getElementById('CF_9814331').value);
}

// functions for getB() and getC()

function useVariables(){
  return getA() + getB() + getC();
}


This is a more acceptable solution because you delay getting the variables until you need them and you also do not have global variables. If the values change you get the updated values as well without having to call getValue() again to 'refresh' your data.