Punched!

Geometry Level 3

The given figure shows a square of side 8 units from which a circular hole of radius 4 units was punched using a punching machine. DF is the diameter of the hole and DFB is a striaght line. The area of the part of the circle which falls outside the sheets is (in square units):

8 π 8 8\pi - 8 8 π 16 8\pi - 16 8 π 4 8\pi - 4 8 π 20 8\pi - 20

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3 solutions

Jason Zou
Jun 30, 2015

We can find the area by finding the area of the semi-circle and subtracting the area of triangle D E G DEG .

The area of the semi-circle is just 1 2 π r 2 = 1 2 π ( 4 ) 2 = 8 π \frac{1}{2}\pi r^2=\frac{1}{2}\pi(4)^2=8\pi .

E G \overline{EG} is a diameter of the circle, so E G = 8 EG=8 . Since triangle D E G DEG is a 4 5 4 5 9 0 45^\circ-45^\circ-90^\circ triangle, E D = D G = 4 2 ED=DG=4\sqrt{2} . Thus, the area of triangle D E G DEG is 1 2 ( 4 2 ) 2 = 16 \frac{1}{2} (4\sqrt{2})^2=16 .

Thus, our desired area is 8 π 16 \boxed{8\pi-16}

Anirban Jana
Mar 21, 2015

EG is the diameter of the circle as ang'DEG is 90 degrees DF & EG being diameters intersect at the center. BY completing tri angle DEG we get an isoceles one with base angles =45 degrees using this find ED =DG { ED cos45= 4} find area of triangle DEG SUBTRACT IT FROM THE AREA OF THE SEMICIRCLE AND there is the ans.......

Caleb Townsend
Mar 11, 2015

Let's call the area we are evaluating "area A A " which is composed of equal areas A 1 A_1 under the square, and A 2 A_2 to the left of the square. Let's also draw E F , F G \overline{EF},\ \overline{FG} forming areas A 3 A_3 above and A 4 A_4 to the right. Finally, let's define area B = 2 A = A 1 + A 2 + A 3 + A 4 , B = 2A = A_1 + A_2 +A_3 + A_4, area C = π r 2 , C = \pi r^2, and area D = s 2 , D = s^2, where s s is the side length of square D E F G . DEFG. A = 1 2 B B = C D = 16 π ( 8 2 ) 2 B = 16 π 32 A = 8 π 16 A = \frac{1}{2}B \\ B = C - D = 16\pi - (\frac{8}{\sqrt{2}})^2 \\ B = 16\pi - 32 \\ \\ A = \boxed{8\pi - 16}

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