Daily logic 7

Geometry Level 3

A man has a rectangular garden, 55m ('B' in the diagram,) by 40m('A',) and he makes a diagonal path, 1m wide, exactly in the manner indicated in the diagram. What is the area of the path?

65 m2 66.67 m2 67 m2 64.8 m2 63.47 m2

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3 solutions

David Vreken
May 8, 2019

Label the path as follows and draw the width of the path intersecting A perpendicular to the path's edges:

Let x = A D x = AD . Then A C = 55 x AC = 55 - x , and since B C = 40 BC = 40 , by Pythagorean's Theorem A B = 4 0 2 + ( 55 x ) 2 AB = \sqrt{40^2 + (55 - x)^2} .

Since D F A B DF || AB , A D E C A B \angle ADE \cong \angle CAB ; and since A E D \angle AED and A C B \angle ACB are right angles, A E D A C B \angle AED \cong \angle ACB . Therefore, A E D A C B \triangle AED \sim \triangle ACB by AA similarity, and A D A E = A B B C \frac{AD}{AE} = \frac{AB}{BC} , or x 1 = 4 0 2 + ( 55 x ) 2 40 \frac{x}{1} = \frac{\sqrt{40^2 + (55 - x)^2}}{40} , which solves to x = 5 3 x = \frac{5}{3} .

The shape of the path is a parallelogram, so when x = 5 3 x = \frac{5}{3} its area is A B A E = 4 0 2 + ( 55 5 3 ) 2 1 = 200 3 66.67 m 2 AB \cdot AE = \sqrt{40^2 + (55 - \frac{5}{3})^2} \cdot 1 = \frac{200}{3} \approx \boxed{66.67 \text{ m}^2} .

The rectangle is 55 40 square meter. Each green triangle is 54 40/2 square meter. The difference is 55 40-2(54 40/2)= 40 square meter for the path ?

John Gutwirth - 2 years, 1 month ago

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Using the diagram above you are assuming AD = 1 m, it is actually AE that equals 1 m. AD is not the width of the path.

Christoph Cox - 2 years, 1 month ago

Longer side of the parallelogram shaped road is 66.66666666...m. Therefore it's area is (1)(66.6666666...)=66.66666..sq. m.

The rectangle is 55 40 square meter. Each green triangle is 54 40/2 square meter. The difference is 55 40-2(54 40/2)= 40 square meter for the path ?

John Gutwirth - 2 years, 1 month ago

40 csc ( θ ) /. Solve [ cot 1 ( 1 40 ( 55 csc ( θ ) ) ) = θ π 8 θ π 2 ] 66 2 3 40 \csc (\theta )\text{/.}\, \text{Solve}\left[\cot ^{-1}\left(\frac{1}{40} (55-\csc (\theta ))\right)=\theta \land \frac{\pi }{8}\leq \theta \leq \frac{\pi }{2}\right] \approx 66\frac23 .

I say approximately as I do not know to simplify the expression solved above to the necessary csc 1 ( 5 3 ) \csc ^{-1}\left(\frac{5}{3}\right) .

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