trapezoid

Geometry Level 3

Which of the following is the closest approximate area of the figure shown above? It is the difference between two isosceles trapezoids whose corresponding sides are parallel.

87 77 57 67

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1 solution

tan θ = 3 8 \tan~\theta = \dfrac{3}{8} \implies θ 22.556 \theta \approx 22.556

cos 20.556 = 3 x \cos~20.556 = \dfrac{3}{x} \implies x = 3 cos 20.556 x=\dfrac{3}{\cos~20.556} \implies x 3.204 x \approx 3.204

By similar figures, we have

3 8 = y 5 \dfrac{3}{8}=\dfrac{y}{5} \implies y = 3 ( 5 ) 8 15 8 = 1.875 y=\dfrac{3(5)}{8}\dfrac{15}{8}=1.875

Let A B A_B be the area of the big trapezoid and A S A_S be the area of the small trapezoid. Then

A B = 1 2 ( 14 + 20 ) ( 8 ) = 136 A_B=\dfrac{1}{2}(14+20)(8)=136

A S = 1 2 ( 9.842 + 13.592 ) ( 5 ) 58.585 A_S=\dfrac{1}{2}(9.842+13.592)(5)\approx 58.585

Finally the area of the figure is 136 58.585 136-58.585 \approx 77 \boxed{77}

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