Area of a rhombus = 1/2 × diagonal 1 × diagonal 2 = 1/2 × 10 × 20 = 100 cm². Choose the correct formula first, substitute the given values carefully, and match the final unit with the type of measurement. A rhombus can be divided by its diagonals into four right triangles. The formula uses both diagonals and then halves the product, so the final answer is measured in square units.