## Explanation

In geometry, the area enclosed by a trapezoid is defined by this formula:

where **a** is the length of side a, **b** is the length of side b, and **h** is the height, measured at a right angle to the base. In Excel, the same formula can be represented like this:

```
=(a+b)/2*h
```

So, for example, to calculate the area of a trapezoid where **a** is 3, **b** is 5, and **h** is 2, you can use a formula like this:

```
=(5+3)/2*2 // returns 8
```

In the example shown, the goal is to calculate the area for eleven trapezoids where **a** comes from column B, **b** is in column C, and **h** comes from column D. The formula in E5 is:

```
=(B5+C5)/2*D5
```

As this formula is copied down the table, it calculates a different area at each new row.

*Note: The parentheses in this formula are optional and for readability only. In Excel's order of operations, multiplication will occur before division, so the parentheses are unnecessary.*