Integrate the function for the curve, as normal, but the change the sign of the result. Be very careful that the curve is always on the same side of the x-axis between the limits of integration. If necessary, partition the integral.
For example, to find the area between the x-axis and sin(x) between x=0 and x=3*pi, you will need
Integral of sin(x) between 0 and pi,
-[integral of sin(x) between pi and 2*pi] - this is where the curve is below the x-axis.
+integral of sin(x) between 2*pi and 3*pi.
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The area under the normal curve is ALWAYS 1.
In statistics you can find the area under a curve to establish what to expect between two input numbers. If there is a lot of area under the curve the graph is tall and there is a higher probability of things occurring there than when the graph is low.
You need to determine the area under the curve between the values in question. This is easy to do because there are tables that give the area values.
The are under the curve on the domain (a,b) is equal to the integral of the function at b minus the integral of the function at a
It is 0.