"lol i did thanx!! but please be my guest how big is the area under the curve f(x)=x ?"
You use integration to solve this. In simple steps, since it's a simple problem.
The area made is in this case a triangle. It has the lenght x and the height f(x)=x. The formula for the area of a triangle is height * lenght / 2.
So, you can just put in values for x and get values for the area out.
Now, in calculus you can do something else, which is rather useful. You can say that a variable *approaches* a value. In our case, we want to know what the area approaches as x approaches infinity.
In this specific case, we can easily see that it approaches infinity. That is, the area in an infinitely big triangle is infinitely large.
Note that this does not mean that all curves gives this answer, but it illustrated easily that math CAN and DO work with infinities, but you have to be careful with them if you want to get something useful out.
I'll give you a small test to see if you can understand the basic concept. Take the function x/2x, and let x go towards infinity. What is the actual value of the function in this case?