Difference between parallel sides (a - b) = 10 cm, Height (h) = 20 cm, Area (A) = 500 cm²

Use the formula $A = \text{average of sides} \times \text{height}$. Since $A = 500$ and $h = 20$, the sum $(a + b)$ is simply $2 \times \frac{A}{h}$, which is $2 \times 25 = 50$.

Students often try to find the individual values of side 'a' and 'b' using the system of equations, which is unnecessary since the question directly asks for the sum (a + b).

This concept is a precursor to coordinate geometry where you might need to calculate the area of a polygon using the coordinates of its vertices.