a. \( S=L W+L H+W H \)

Did you know that this formula \( S = L W + L H + W H \) is used to calculate the surface area of a rectangular box? It’s derived from the concept of finding the area of each of the three pairs of opposite faces and summing them up. This is super useful in real-world applications, like when you want to know how much paint you need to cover a box or if you're packing materials! To really nail down this formula, be careful with the dimensions you’re using—the length \( L \), width \( W \), and height \( H \) should all be in the same unit, like meters or inches. A common mistake is mixing units, which can lead to some puzzling results! So, always double-check before diving into calculations. Happy measuring!