Ninja Turtle!

Geometry Level 2

When sketching out Leonardo Da Turtle, I started out with a green circle of radius 2, and then added a blue band across his eyes. The circular arcs of the blue band each subtend an angle of 60 degress at the center. And 2 triangles to fill in the gap. Finally, I added eyes of diameter 1.

What is the (visible) area of the blue band?

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

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5 solutions

Without considering the eyes of the ninja turtle (two circles), the area of the blue region is composed of two circular sectors with central angle of 6 0 60^\circ and two isosceles triangles (it has two equal sides of length 2 2 and an included angle of 12 0 120^\circ ). So the area of the visible blue region is equal to the area of the blue region minus the area of the two circles. We have

A = 2 ( 30 360 ) ( π ) ( 2 2 ) + 2 ( 1 2 ) ( 2 ) ( 2 ) ( sin 120 ) π 4 ( 1 2 ) = 4 3 π + 4 ( 3 2 ) π 2 = 4 3 π + 2 3 π 2 = 5 6 π + 2 3 A=2\left(\dfrac{30}{360}\right)(\pi)(2^2)+2\left(\dfrac{1}{2}\right)(2)(2)(\sin 120)-\dfrac{\pi}{4}(1^2)=\dfrac{4}{3}\pi+4\left(\dfrac{\sqrt{3}}{2}\right)-\dfrac{\pi}{2}=\dfrac{4}{3}\pi + 2\sqrt{3}-\dfrac{\pi}{2}=\boxed{\dfrac{5}{6}\pi +2\sqrt{3}}

Notes:

  1. area of a circular sector = θ 360 π r 2 \text{area of a circular sector =}\dfrac{\theta}{360}\pi r^2 where θ \theta is the central angle and r r is the radius

  2. area of a triangle = 1 2 a b sin C \text{area of a triangle =} \dfrac{1}{2}ab \sin C where a a and b b are two adjacent sides and C C is the included angle

  3. area of a circle given diameter = π 4 d 2 \text{area of a circle given diameter}=\dfrac{\pi}{4} d^2 where d d is the diameter.

  4. sin 120 = 3 2 \sin 120 = \dfrac{\sqrt{3}}{2}

Hey buddy the angle is 60 degrees not 30. Might be a tempo but just saying.

इश्वर बस्याल - 3 years, 6 months ago
Austin Cheng
Apr 16, 2015

We can divide the blue band into two congruent sectors and two congruent triangles by drawing two diameters as shown above.

Area of sector = 60 360 × π ( 2 ) 2 \frac{60}{360} \times π(2)^{2} = 2 3 π \frac{2}{3}π

There are two of these so the area of both the sectors combined is 4 3 π \frac{4}{3}π

Area of triangle = 1 2 × 2 × 2 × sin 120 ° \frac{1}{2} \times 2 \times 2 \times \sin 120° = 3 \sqrt{3}

There are two of these so the area of both the triangles combined is 2 3 2\sqrt{3}

So the area of the blue band is 4 3 π + 2 3 \frac{4}{3}π + 2\sqrt{3}

Now we have to subtract the areas of the two smaller circles.

Area of smaller circle = π ( 1 2 ) 2 π (\frac{1}{2})^{2} = 1 4 π \frac{1}{4}π

There are two of these so the area of both the smaller circles combined is 1 2 π \frac{1}{2}π

So the area of the visible blue band is 4 3 π + 2 3 1 2 π \frac{4}{3}π + 2\sqrt{3} - \frac{1}{2}π = 5 6 π + 2 3 \boxed {\frac{5}{6}π + 2\sqrt{3}}

Great thanks!

Chung Kevin - 6 years, 1 month ago

I solved without using trigonometry.

Scott Rodham
Mar 16, 2017

Blue area = Large circle - 2 green segments - 2 white circles

= 4π - 2(4π/3 - √3) -π/2

= 5π/6 + 2√3

Dillon Chew
Apr 16, 2015

Area of 2 sectors = (πr^2)/3 since 60 degrees sector. Area of small circles (eyes) = 2 x π x (1/2)^2 = 1/2π Area of 2 triangles = 2*sqrt(3). Use trigonometry. Visible Blue Area = 4/3π-1/2π +2/3 = 5/6π + 2/3

Can you provide a Detailed Solution? Or possibly a solution without Trigonometry?

Mehul Arora - 6 years, 1 month ago

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Area of each of the two 6 0 o s e c t o r s = π 6 2 2 . Each of the two triangle fill in the gap are 30-60-90 right angled, with the hypotenuse= 2. So area of each is 1 2 1 3 . . So half blue area + an eye = π 6 2 2 + 2 1 2 1 3 H a l f t h e r e q u i r e d a r e a = a b o v e h a l f a r e a a n e y e = π 6 r 2 + 1 2 1 3 π ( 1 2 ) 2 = 5 12 + 3 . \text{Area of each of the two } 60^o~ sectors =\dfrac \pi 6 *2^2.\\\text{Each of the two triangle fill in the gap are 30-60-90 right angled,}\\\text{with the hypotenuse= 2. So area of each is }\dfrac 1 2 * 1*\sqrt3..\\\text{So half blue area + an eye }=\dfrac \pi 6 *2^2 +2*\dfrac 1 2 * 1*\sqrt3\\\therefore~Half~ the~ required~ area~=above~ half~area - an ~eye\\ =\dfrac \pi 6 *r^2+\dfrac 1 2 * 1*\sqrt3-\pi*(\dfrac 1 2 )^2=\color{#3D99F6}{\dfrac {5}{12} +\sqrt 3} .

Niranjan Khanderia - 6 years, 1 month ago

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Another way to deal with the 2 triangles is to notice that when you combine them along their longest end, you get 2 equilateral triangles.

Chung Kevin - 6 years, 1 month ago

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@Chung Kevin Thanks. It is a simpler way than mine.

Niranjan Khanderia - 6 years, 1 month ago

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@Niranjan Khanderia I must report to this question since no triangle was made earlier

Uttkarsh Singh - 5 years, 2 months ago

How do you get from a 30-60-90 triangle with hypotenuse of 2 to area of 1/2 * 1 * 3^1/2 ?

Dean Hale - 4 years, 6 months ago

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