Geometry and Trigonometry practice question

Answer: 54

The area of a trapezoid is A = ½(b₁ + b₂)h = ½(7 + 11)(6) = ½(18)(6) = 54 square centimeters. Common errors this item targets: omitting the factor of ½ and reporting 108; multiplying only one base by the height (7 × 6 = 42 or 11 × 6 = 66); or averaging the bases and forgetting to multiply by the height (9).

Common wrong answers

If you answered 9

You correctly averaged the bases to get 9, but stopped before multiplying by the height of 6: 9 × 6 = 54.

If you answered 42

You multiplied only the shorter base by the height (7 × 6). The trapezoid formula uses the average of both bases: ½(7+11)(6) = 54.

If you answered 66

You multiplied only the longer base by the height (11 × 6). Use the average of the two bases instead: ½(7+11)(6) = 54.

If you answered 108

You added the bases and multiplied by the height but never took half: (7+11)(6) = 108, so divide by 2 to get 54.

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