Please help me how could i insert an input number to be used in a formula to get the result without me myself calculating it each time . how can I do so .
Hi,
Welcome to the forum.
Please read the first post in any forum entitled how to use this forum.
http://forum.arduino.cc/index.php/topic,148850.0.html
You can program the formula into the controller if it does not rely on super accuracy, but you cannot just write a formula in the IDE and make it work.
You need a controller to do the work.
Use the serial monitor on the IDE to input and output your data.
Tom.. 
Can you give us an example of what you're trying to do? The Arduino can do math at runtime, and the IDE (compiler) pre-processor can do some math too.
You'll need to write an expression with the "answer" on the left like this:
X = 1 + 2;
Or,
X = Y +3; // Where Y has already been defined.
And, you can do things that don't make sense in "regular math" like:
X = X + 1;