Ant highway

Geometry Level 5

Imagine the square grid is a network of narrow ant highways spaced 1 m e t e r 1 meter apart.

An ant can walk 1 m e t e r 1 meter in 15 s e c o n d s 15 seconds along these roads. However, if an ant strays off of a highway and into the dirt, it can only travel half as fast.

The four colored paths lead to points an ant can reach in 30 s e c o n d s 30 seconds .

Find the area in m 2 m^{2} of the region composed of all points an ant can reach in at most 30 s e c o n d s 30 seconds .

Hint: the optimum direction for heading off the highway is 60 d e g r e e s 60 degrees .


The answer is 6.784609691.

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3 solutions

Michael Mendrin
Jul 23, 2018

Jeremy Galvagni
Jul 23, 2018

This image is the set of all points an ant can reach in 30 seconds. It has enough symmetry that we can analyze half of one quadrant and multiply the resulting area by 8.

Beginning with A B C \triangle ABC we have A B = 1 = h + h 3 AB=1=h+h\sqrt{3} so h = 3 1 2 h=\frac{\sqrt{3}-1}{2} and the entire light and dark green region has area 3 1 2 \frac{\sqrt{3}-1}{2}

For the red, orange and yellow regions it will help to coordinatize. D = ( 0 , 0 ) D=(0,0) , A = ( 1 , 0 ) A=(1,0) , B = ( 2 , 0 ) B=(2,0) , I = ( 1 , 1 ) I=(1,1)

The line containing D I DI is y = x y=x . The line containing H I HI is y = 3 x + 1 3 y=\sqrt{3}x+1-\sqrt{3} . The line containing G H GH and also B C BC is y = 3 3 x + 2 3 3 y=\frac{-\sqrt{3}}{3}x+\frac{2\sqrt{3}}{3}

From which we can solve for the points of intersection. G = ( 3 1 , 3 1 ) G=(\sqrt{3}-1,\sqrt{3}-1) and H = ( 5 3 4 , 1 + 3 4 H=(\frac{5-\sqrt{3}}{4},\frac{1+\sqrt{3}}{4}

The area of D E G = 1 2 ( 3 1 ) ( 3 1 ) = 2 3 \triangle DEG=\frac{1}{2}(\sqrt{3}-1)(\sqrt{3}-1)=2-\sqrt{3}

The area of trapezoid E F H G = 1 2 ( 5 3 4 ( 3 1 ) ) ( 3 1 4 + 3 1 ) = 1 16 ( 51 + 30 3 ) EFHG=\frac{1}{2}(\frac{5-\sqrt{3}}{4}-(\sqrt{3}-1))(\frac{\sqrt{3}-1}{4}+\sqrt{3}-1)=\frac{1}{16}(-51+30\sqrt{3})

The area of trapezoid F A I H = 1 2 ( 1 5 3 4 ) ( 1 + 1 + 3 4 ) = 1 16 ( 2 3 1 ) FAIH=\frac{1}{2}(1-\frac{5-\sqrt{3}}{4})(1+\frac{1+\sqrt{3}}{4})=\frac{1}{16}(2\sqrt{3}-1)

Sum the four areas and multiply the result by 8 8 gives the final answer 12 3 14 6.784609691 12\sqrt{3}-14 \approx \boxed{6.784609691}

Iliya Hristov - 11 months, 4 weeks ago
Vinod Kumar
Aug 6, 2018

I just find the relevant equations of the lines in the positive (x-y) quadrants, whose intersections determine the areas in three squares along x and y axis. In the square (1) the lines are, x/(2/√3)+y/2=1, y=x, 3y-√3x=(3-√3), and symmetry along line (y=x). The area = {(5/√3)-2} + {(1/√3)-(1/2)}= 0.964101, while in other two squares (2) additional line is y=x+1 symmetry line and area= {1/(1+√3)}= 0.366025 and (3) same as in (2) with area= {1/(1+√3)}=0.366025. Thus in the (x-y) quadrant area=1.696151 and total area in all four quadrants gives Answer=4*1.696151=6.7846.

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