There are several huge issues with the form...first, you call the function "calculate", but in the HTML form, you call the function "CalculateSum". so that won't work. Secondly, in the function, you have 5 "options" that need to be included (Atext,Btext,Ctext,Dtext,E, form), but when you call the function in the form, you only give 4 options.
Next, in the function itself, you end the function after the "E = [(Atext + Btext)- Ctext] / Dtext;" line, so the following code doesn't get executed, and doesn't output anything.
Also, you have things in the function that have no bearing on anything.
This does nothing, since you aren't accessing the function through a variable.
Code:
var A = parseFloat(Atext);
var B = parseFloat(Btext);
var C = parseFloat(Ctext);
var D = parseFloat(Dtext);
While this does something, it's not used anywhere else, so what's the point of this?
A quick scan of the code blocks shows a myriad of errors. Correct these and see if that doesn't help you.