Xor Gate?

Geometry Level 2

Two unit circles intersect each other at two points, and they pass through each other's centers as shown above. What is the area of the shaded region?

2 π + 6 3 6 \frac{2\pi + 6\sqrt3}6 3 π + 6 3 6 \frac{3\pi + 6\sqrt3}6 4 π + 6 3 6 \frac{4\pi + 6\sqrt3}6 5 π + 6 3 6 \frac{5\pi + 6\sqrt3}6

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5 solutions

Hung Woei Neoh
Jul 7, 2016

Relevant wiki: Length and Area - Composite Figures

Notice that lines O A , O P , O B , A P OA,OP,OB,AP and B P BP are all radii of the unit circle, meaning they have a length of 1 1 .

Since all the lines have the same length, we can say that O A P \triangle OAP and O B P \triangle OBP are equilateral triangles.

Now, notice that the area of the shaded region is given as below:

Therefore, we need to calculate each component first.

Area of unit circle = π ( 1 ) 2 = π =\pi(1)^2=\pi

Area of sector O A P B = 120 360 × π ( 1 ) 2 = π 3 OAPB = \dfrac{120}{360}\times \pi(1)^2= \dfrac{\pi}{3}

Area of O A P = 1 2 ( 1 ) 2 ( sin 6 0 ) = 1 2 ( 3 2 ) = 3 4 \triangle OAP = \dfrac{1}{2}(1)^2(\sin60^{\circ}) = \dfrac{1}{2}\left(\dfrac{\sqrt{3}}{2}\right)=\dfrac{\sqrt{3}}{4}

Area of shaded region

= 2 ( =2\Big( Area of unit circle 2 × -2 \times Area of sector O A P B + 2 × OAPB+2\times Area of O A P ) \triangle OAP\Big)

= 2 ( π 2 × π 3 + 2 × 3 4 ) = 2 ( π 3 + 3 2 ) = 2 π 3 + 3 = 2 π + 3 3 3 = 4 π + 6 3 6 =2\left(\pi - 2 \times \dfrac{\pi}{3} + 2 \times \dfrac{\sqrt{3}}{4}\right)\\ =2\left(\dfrac{\pi}{3} + \dfrac{\sqrt{3}}{2}\right)\\ =\dfrac{2\pi}{3}+\sqrt{3}\\ =\dfrac{2\pi+3\sqrt{3}}{3}\\ =\boxed{\dfrac{4\pi+6\sqrt{3}}{6}}

Hana Wehbi
Jul 7, 2016

Relevant wiki: Length and Area - Composite Figures

B a n d C B \ and \ C are the centers of our circles. Triangles A B C a n d B C D ABC \ and\ BCD are equilateral triangles with each angle=60.

We know the mentioned sides that are equal to the radius. With the application of Pythagoras, we get A D = 3 r AD= \sqrt{3}r and angle A B D = 120 ABD=120 \implies the area of the intersection of the two circles(the one highlighted in yellow) is r 2 × ( 2 π 3 3 2 ) r^2\times(\frac{2\pi}{3}-\frac{\sqrt{3}}{2}) = 2 [ r 2 2 ( 2 π 3 s i n ( 2 π 3 ) ) ] 2[\frac{r^2}{2}(\frac{2\pi}{3}-sin(\frac{2\pi}{3}))] .The area of the two circles is 2 π r 2 2\pi r^2 .

Thus, the area of one white part or one green part in our problem is π r 2 r 2 × ( 2 π 3 3 2 ) = 2 π + 3 3 6 \pi r^2- r^2\times(\frac{2\pi}{3}-\frac{\sqrt{3}}{2})= \frac{2\pi+3\sqrt{3}}{6} . Keeping in mind that r = 1 r=1 . Multiply this area by 2 2 , we obtain 2 π + 3 3 3 \boxed{\Large\frac{2\pi+3\sqrt{3}}{3}} = 4 π + 6 3 6 \boxed{\Large\frac{4\pi+6\sqrt{3}}{6}}

Notice that when you calculate the area of one unit circle, you added the intersection area once.

Therefore, when you add the area of two unit circles, you added the intersection area twice

I made that same mistake too

Hung Woei Neoh - 4 years, 11 months ago

Relevant wiki: Circles - Area

Consider figure A A , the distance between the centers of the circle is 1 1 .

Consider figure B B , the area of one shaded part is equal to the area of a circle minus the area of two segments of a circle. We need the value of θ \angle \theta ,

cos θ = 1 2 1 = 1 2 \cos \theta = \dfrac{\frac{1}{2}}{1}=\dfrac{1}{2}

θ = cos 1 1 2 = 60 \theta = \cos^{-1}\dfrac{1}{2} = 60 .

It follows that 2 θ = 120 2 \theta = 120 .

We need the value of a a , by pythagorean theorem

a = 1 ( 1 2 ) 2 = 3 4 = 3 2 a=\sqrt{1-\left(\dfrac{1}{2}\right)^2}=\sqrt{\dfrac{3}{4}}=\dfrac{\sqrt{3}}{2}

It follows that 2 a = 3 2a=\sqrt{3}

Consider figure C C . The area of one segment is 120 360 π 1 2 ( 1 2 ) ( 3 ) = π 3 3 4 \dfrac{120}{360} \pi - \dfrac{1}{2}\left(\dfrac{1}{2}\right)(\sqrt{3})=\dfrac{\pi}{3}-\dfrac{\sqrt{3}}{4} . So the area of two segments is 2 ( π 3 3 4 ) = 2 π 3 3 2 2\left(\dfrac{\pi}{3}-\dfrac{\sqrt{3}}{4}\right)=\dfrac{2 \pi}{3}-\dfrac{\sqrt{3}}{2}

So the area of one shaded part is π ( 2 π 3 3 2 ) = π 2 π 3 + 3 2 = 2 π + 3 3 6 \pi -\left(\dfrac{2 \pi}{3}-\dfrac{\sqrt{3}}{2}\right)=\pi - \dfrac{2 \pi}{3} + \dfrac{\sqrt{3}}{2}=\dfrac{2\pi + 3\sqrt{3}}{6} .

Finally, the area of two shaded part is 2 ( 2 π + 3 3 6 ) = 4 π + 6 3 6 2\left(\dfrac{2\pi + 3\sqrt{3}}{6}\right) = \boxed{\dfrac{4 \pi + 6\sqrt{3}}{6}}

In second last step, why didn’t you subtract area of two segments directly from combined area of two circles? It should have given you the same answer right. I actually did but my answer came out different. Please help

Atharva Vankundre - 6 months ago
Marta Reece
Apr 19, 2017

The white area in the center, A w A_w , is two circular segments, each with angle 12 0 120^\circ and radius 1 1 .

A w = 2 × R 2 2 ( α π 180 sin α ) = 2 × 1 2 ( 120 π 180 sin 12 0 ) = 2 π 3 3 2 A_w=2\times\frac{R^2}{2}(\frac{\alpha\pi}{180}-\sin\alpha)=2\times\frac{1}{2}(\frac{120\pi}{180}-\sin120^\circ)=\frac{2\pi}{3}-\frac{\sqrt{3}}{2}

Green area on the left is area of circle minus the white area: π ( 2 π 3 3 2 ) = π 3 + 3 2 \pi-(\frac{2\pi}{3}-\frac{\sqrt{3}}{2})=\frac{\pi}{3}+\frac{\sqrt{3}}{2}

Total green area is double that. 2 × ( π 3 + 3 2 ) = 2 π 3 + 3 = 4 π + 6 3 6 2\times(\frac{\pi}{3}+\frac{\sqrt{3}}{2})=\frac{2\pi}{3}+\sqrt{3}=\frac{4\pi+6\sqrt{3}}{6}

Daniel Turizo
Jul 7, 2016

OP can you post a solution? I apologize but I'm not sure about the answer. In my opinion the answer should be 4 π + 6 3 6 \frac{4 \pi + 6 \sqrt{3}}{6} , and I would like to see where I am wrong. Thanks.

No, you're right, see my solution

And if you have any issues with the solution, do post a report instead in the future

Hung Woei Neoh - 4 years, 11 months ago

Thanks. I've updated the answer from 8 π + 3 3 6 \frac{8\pi + 3\sqrt3}6 to 4 π + 6 3 6 \frac{4\pi + 6\sqrt3}6 . Those who previously attempted this problem will be marked correct.

In future, if you spot any errors with a problem, you can “report” it by selecting "report problem" in the menu. This will notify the problem creator (and eventually staff) who can fix the issues.

Brilliant Mathematics Staff - 4 years, 11 months ago

Sorry guyyyyyys!

Pi Han Goh - 4 years, 11 months ago

0 pending reports

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