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?
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Hey buddy the angle is 60 degrees not 30. Might be a tempo but just saying.
We can divide the blue band into two congruent sectors and two congruent triangles by drawing two diameters as shown above.
Area of sector = 3 6 0 6 0 × π ( 2 ) 2 = 3 2 π
There are two of these so the area of both the sectors combined is 3 4 π
Area of triangle = 2 1 × 2 × 2 × sin 1 2 0 ° = 3
There are two of these so the area of both the triangles combined is 2 3
So the area of the blue band is 3 4 π + 2 3
Now we have to subtract the areas of the two smaller circles.
Area of smaller circle = π ( 2 1 ) 2 = 4 1 π
There are two of these so the area of both the smaller circles combined is 2 1 π
So the area of the visible blue band is 3 4 π + 2 3 − 2 1 π = 6 5 π + 2 3
Great thanks!
I solved without using trigonometry.
Blue area = Large circle - 2 green segments - 2 white circles
= 4π - 2(4π/3 - √3) -π/2
= 5π/6 + 2√3
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?
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Area of each of the two 6 0 o s e c t o r s = 6 π ∗ 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 2 1 ∗ 1 ∗ 3 . . So half blue area + an eye = 6 π ∗ 2 2 + 2 ∗ 2 1 ∗ 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 = 6 π ∗ r 2 + 2 1 ∗ 1 ∗ 3 − π ∗ ( 2 1 ) 2 = 1 2 5 + 3 .
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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.
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@Chung Kevin – Thanks. It is a simpler way than mine.
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@Niranjan Khanderia – I must report to this question since no triangle was made earlier
How do you get from a 30-60-90 triangle with hypotenuse of 2 to area of 1/2 * 1 * 3^1/2 ?
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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 ∘ and two isosceles triangles (it has two equal sides of length 2 and an included angle of 1 2 0 ∘ ). 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 ( 3 6 0 3 0 ) ( π ) ( 2 2 ) + 2 ( 2 1 ) ( 2 ) ( 2 ) ( sin 1 2 0 ) − 4 π ( 1 2 ) = 3 4 π + 4 ( 2 3 ) − 2 π = 3 4 π + 2 3 − 2 π = 6 5 π + 2 3
Notes:
area of a circular sector = 3 6 0 θ π r 2 where θ is the central angle and r is the radius
area of a triangle = 2 1 a b sin C where a and b are two adjacent sides and C is the included angle
area of a circle given diameter = 4 π d 2 where d is the diameter.
sin 1 2 0 = 2 3