Need help with an Algebra calculation

I have written a program to calculate the area under a curve, using the area of a trapezium rule. As there are two values that vary and not one, the integration rule wont work. So I want to work out each trapezium's area and then sum them.

I have set up two arrays, x and y. The user enters the values into x and then y. Now i need to make the calculation using the following rule, using i as the current value:

((x[i+1]-x[i])*(y[i]+y[i+1]))/2

This gives the area for each trapezium. Then I need to sum them all together to give a total area and thus the area under a curve.

P.S. the area of a trapezium:

Area = (interval width * positive difference in heights)/2

Can anyone help me with the for statements to do this please?

EDIT:

the area of a trapezium SHOULD read:

Area = (width * (height left + height right))/2

You want to add the values up while you run along the for loop. It is very inefficient to actually store the values and then add them up later.

Code:

---

int numValues = the number of values
int i;
float area = 0;

// numValues - 1, for example, the points (0, 0), (1, 3), (3, 1)
//  represent two trapeziums.
for (i = 0; i < numValues - 1; i++)
{
 area += ((x[i+1]-x[i])*(y[i]+y[i+1]))/2
}
return area;

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