Find This Area

Geometry Level 2

In the picture, the arc AC is one fourth of a circumference of center D and the arc AB is one eighth of a circumference of center C. The segment AD has a length of 2 cm. What is the area in cm² of the green region?

Problem from OBMEP 2017.

4 π 4\pi 4 2 2 π 2\pi π \pi

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2 solutions

Since arc A C AC is one fourth of circumference of center D D , then C D A = 9 0 \angle CDA=90^\circ . Since arc A B AB is 1 8 \dfrac{1}{8} of circumference of center C C , A C B = 4 5 \angle ACB=45^\circ .

By Pythagorean theorem, A C = 2 2 + 2 2 = 8 = 2 2 AC=\sqrt{2^2+2^2}=\sqrt{8}=2\sqrt{2} .

It follows that B C = A C = 2 2 BC=AC=2\sqrt{2} .

Consider sector C D A : CDA: The area of sector C D A CDA is A = 1 4 π ( 2 2 ) = π A=\dfrac{1}{4}\pi (2^2)=\pi

Consider sector A C B : ACB: The area of sector A C B ACB is A = 45 360 π ( 2 2 ) 2 = 1 8 π ( 4 ) ( 2 ) = π A=\dfrac{45}{360}\pi(2\sqrt{2})^2=\dfrac{1}{8}\pi(4)(2)=\pi

Area of the yellow segment is π 1 2 ( 2 ) ( 2 ) = π 2 \pi-\dfrac{1}{2}(2)(2)=\pi-2

Finally, the area of the green region is π ( π 2 ) = π π + 2 = \pi-(\pi-2)=\pi-\pi+2= 2 \boxed{2}

Unshaded area:

= 1 8 ( 2 2 ) 2 π ( 1 4 2 2 π 1 2 2 2 ) = 1 8 8 π ( 1 4 4 π 2 ) = π ( π 2 ) = π π + 2 = 2 =\frac { 1 }{ 8 } { \left( 2\sqrt { 2 } \right) }^{ 2 }\pi -\left( \frac { 1 }{ 4 } \cdot { 2 }^{ 2 }\pi -\frac { 1 }{ 2 } \cdot { 2 }^{ 2 } \right) \\ =\frac { 1 }{ 8 } \cdot 8\pi -\left( \frac { 1 }{ 4 } \cdot 4\pi -2 \right) \\ =\pi -(\pi -2)\\ =\pi -\pi +2\\ =2

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