In the dark part 2

Geometry Level 3

Four isosceles right triangles with legs of length 3 are inside a circle of radius 5 and positioned on a unit grid.

Consider every possible chord of the circle that does not touch a triangle. Then shade every portion of the white inside the circle that is not touched by any of such chords.

The shaded area is a b , \frac{a}{b}, where a a and b b are coprime positive integers.

What is a + b ? a+b?


The answer is 89.

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3 solutions

Jeremy Galvagni
Nov 17, 2018

The 4 congruent blue triangles are the only points that should be shaded. Thanks to @Henry U for pointing out the triangles such as the yellow one should not be shaded.

Now, let's take a closer look:

If we coordinatize as shown, the lines that form one triangle are bounded by the lines x = 0 x=0 , y = x / 4 y=x/4 , and y = 3 4 x y=3-4x and the vertices are shown.

The area of one triangle is then 1 2 3 12 17 = 18 17 \frac{1}{2} \cdot 3 \cdot \frac{12}{17} = \frac{18}{17} . The total area is 4 18 17 = 72 17 4 \cdot \frac{18}{17} = \frac {72}{17} and the answer is 72 + 17 = 89 72+17 = \boxed{89}


Note: I accidentally mislabelled the point (0,3) as (3,0). It does not affect the result.

You should specify that the isosceles triangles aren't shaded.

Abraham Zhang - 2 years, 6 months ago

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It would also be nice if the wording was clearer as to what one should be solving for, especially the "then shade every portion of the white..." part.

Yannis Wu-Yip - 2 years, 6 months ago

Could yoh please tell me why the triangles shaded in yellow should not be included?

Biplob Manna - 2 years, 6 months ago

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There are lines through the center of the circle that pass through the yellow.

Jeremy Galvagni - 2 years, 6 months ago

little issue with your diagram, the point (12/17,3/17) should be (3/17,12/17) or (3,0) should be (0,3). The latter probably since you took y=3-4x

Blop Néel - 2 years, 6 months ago

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Thanks for the correct. I make that mistake so often with points on the x or y-axis. I'll add a note.

Jeremy Galvagni - 2 years, 6 months ago

Where can I learn more about this and what would it be called?

Andrew Cubero - 2 years, 6 months ago
Binky Mh
Nov 26, 2018

We are left with 4 4 triangles which we need to find the size of. We can do this by looking at similar triangles:

By comparing to the larger triangle, we can see that it has an edge ratio of 1 : 4 1:4 , and a hypotenuse of length 3 3 . By using Pythag, we can get the length of the smaller side l l : l 2 + ( 4 l ) 2 = 3 2 l 2 + 16 l 2 = 9 17 l 2 = 9 l = 9 17 \begin{aligned}l^2+(4l)^2&=3^2\\ l^2+16l^2&=9\\ 17l^2&=9\\ l\:&=\sqrt{\frac{9}{17}}\end{aligned} And now we can find the area: A = l × 4 l 2 = 2 l 2 A = 2 × 9 17 A = 18 17 \begin{aligned}A&=\frac{l\times4l}{2}=2l^2\\ A&=2\times\frac{9}{17}\\ A&=\frac{18}{17}\end{aligned} And as there are 4 4 of these triangles, we can find the total area 4 × 18 17 = 72 17 4\times\frac{18}{17}=\frac{72}{17} giving us our final answer of 72 + 17 = 89 72+17=\boxed{89} .

Anyone know how to make the images not ridiculously large?

Binky MH - 2 years, 6 months ago
Vinod Kumar
Nov 27, 2018

Determine three lines in the positive quadrant: (1) line joining (0,1) and (1,-3), y=-4x+1,

(2) line joining (-1,0) and (3,1), y=(x+1)/4,

(3) line joining (0,1) and (3,1), y=1.

These three lines create one internal triangle surrounded by all three straight lines.

The three vertices of this triangle in the positive quadrant are determined by intersection of the above three lines as (0,1), (3,1) and (3/17,5/17). The area of this right triangle is = (18/17).

There is an identical triangle in all the four quadrants.

Total area =(4*18)/17 = 72/17,

Answer=72+17=89

What about the chords outside the triangles? ... Such as from 9 o’clock on the circle to 12 o’clock?

Fred Litt - 2 years, 6 months ago

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Jeremey Galvagni's Diagram contains two types of triangles, blue and yellow. Blue triangle area is 18/17 and yellow triangle area is 9/10.

However, as explained by Binky Mh and Jeremey Galvagni in his comments, the yellow triangles are excluded, because, the chords passing through center are covering yellow triangles.

Therefore, only blue triangles with total area 4*(18/17) are not covered by chords.

Vinod Kumar - 2 years, 6 months ago

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