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jmc

geometry intermediate

Problem

is a regular octagon of side 12cm. Find the area in square centimeters of trapezoid . Express your answer in simplest radical form.

problem
Solution
Let the perpendiculars from and to intersect at and , respectively. These perpendiculars split trapezoid into two isosceles right triangles and and one rectangle .

In isosceles right triangles (which have angles 45-45-90), the ratio of the leg length to the hypotenuse length is ; hence, we have . We also have , as opposite sides of a rectangle are equal.

Thus, trapezoid has bases of length and , and height of length . Hence its area is .
Final answer
72 + 72\sqrt{2}