Thursday, December 24, 2015

Church Window Design

Last week while I was waiting to pick up my kids from their school carpool line, I noticed that all the windows in the church building had a certain design.  Despite being different widths and heights, each window was symmetrical, where both left and right sides curved up to a point, the outermost curves continued on to meet at another point, and there was a circle in the top middle that was just the right size to perfectly touch all the other curves in one spot each.


I began to wonder about that middle circle.  How did the architect find the correct size of that circle so that it was tangential to the other curves?  Was it by trial and error?  Or was there a formula for it?  Could a general formula be found for it?

To simplify things, let’s assume that all the curves were made by circles (and not by some other conic section like a parabola, ellipse, or hyperbola).  The four different curves can then be constructed by using four congruent circles as guidelines. 


Each window can then be defined by two variables: the radius r of the four congruent circles, and the distance w between the two nonsymmetrical arcs (or half the width of the whole window).  Since each window design is symmetrical, the center of the middle circle with the unknown radius x will lie on the vertical line of symmetry.  We can now construct two right triangles with the following sides as pictured below:


The left-side triangle has a hypotenuse of r – x and legs r – w and y, which makes the Pythagorean equation y2 + (r – w)2 = (r – x)2 or y2 = (r – x)2 – (r – w)2.  The right-side triangle has a hypotenuse of r + x and legs r and y, which makes the Pythagorean equation y2 + r2 = (r + x)2 or y2 = (r + x)2 – r2.  Therefore,

y2 = (r – x)2 – (r – w)2 = (r + x)2 – r2
(Pythagorean’s Theorem)
(r2–2rx+x2) – (r2–2rw+w2) = (r2+2rx+x2) – r2
(expand)
-2rx + x2 + 2rw – w2 = 2rx + x2
(simplify)
-2rx + 2rw – w2 = 2rx
(subtract x2)
2rw – w2 = 4rx
(add 2rx)
x = (2rw – w^2)/4r
(divide 4r)

The radius x of the middle circle is therefore


where r is the radius of one of the larger four congruent circles and w is half the width of the whole window.  So if the radius of one of the larger circles is 3 feet, and half the width of the window is 2 feet, then the radius of the middle circle would be x = (2rw – w^2)/4r = (2(3)2 – 2^2)/4(3) = 8/12 feet or 8 inches.  Or if the radius of one of the larger circles is 8 feet, and half the width of the window is 4 feet, then the radius of the middle circle would be x = (2rw – w^2)/4r = (2(8)4 – 4^2)/4(8) = 48/32 = 3/2 feet or 18 inches.

As a side note, calculus lovers should recognize that if r remains constant, the maximum radius x of the middle circle occurs when w = r.  This is because dx/dw = 1/4r(2r – 2w), and setting this to zero gives the solution w = r.  Geometrically, w = r occurs when the top of the window does not come to a point but is a smooth semicircle.

There are probably other church window designs that have similar properties.  For this particular design, the size of the middle circle can be calculated exactly.

Tuesday, December 1, 2015

Kissing Circles (Three Circles and a Line)

Circles that are mutually tangent to each other are called “kissing circles” because they barely touch each other (or “kiss”) at one point.  There are several different diagrams that have kissing circles, but this article will focus on the specific case of three circles and a line that are all mutually tangent to each other, as in the diagram below:


When three circles and a line are mutually tangent to each other, the relationship between the three radii of each circle is


The proof can be found by solving the three right triangles formed by using the segments from one center of a circle to the next as hypotenuses, as pictured below in gray:


The bottom left gray triangle makes the Pythagorean equation (r1 – r3)2 + x12 = (r1 + r3)2, which simplifies to

(r1 – r3)2 + x12 = (r1 + r3)2
(Pythagorean’s Theorem)
r12 – 2r1r3 + r32 + x12 = r12 + 2r1r3 + r32
(expand)
-2r1r3 + x12 = 2r1r3
(subtract r12 and r32)
x12 = 4r1r3
(add 2r1r3)
x1 = 2√(r1r3)
(square root)

The bottom right gray triangle makes the Pythagorean equation (r2 – r3)2 + x22 = (r2 + r3)2, which similarly simplifies to x2 = 2√(r2r3), and the top gray triangle makes (r1 – r2)2 + x32 = (r1 + r2)2, which simplifies to x3 = 2√(r1r2).  Since x1 + x2 = x3,

x1 + x2 = x3
(angle addition)
2√(r1r3) + 2√(r2r3) = 2√(r1r2)
(substitution)
√(r1r3) + √(r2r3) = √(r1r2)
(divide by 2)
√r3(√r1 + √r2) = √(r1r2)
(factor √r3)
√r3 = √(r1r2)/(√r1 + √r2)
(divide by (√r1 + √r2))
1/√r3 = (√r1 + √r2) /√(r1r2)
(rearrange)
1/√r3 = √r1/√(r1r2) + √r2/√(r1r2)
(distribute denominator)
1/√r3 = 1/√r2 + 1/√r1
(simplify)
1/√r1 + 1/√r2 = 1/√r3
(rearrange)

The mathematician Descartes studied the case of four kissing circles and derived the equation


where k1 = 1/r1, k2 = 1/r2, k3 = 1/r3, and k4 = 1/r4.

Four Kissing Circles

The derivation for this formula involves solving a quadratic system of three equations of three variables and is beyond the scope of this article.  However, we can use Descartes’ Theorem to as another proof for 1/√r1 + 1/√r2 = 1/√r3, because the case of three kissing circles and a line is actually a specific case in which the line has a curvature of k3 = 0.  Therefore,

k4 = k1 + k2 + k3 ± 2√(k1k2 + k2k3 + k1k3)
(Descartes’ Theorem)
k4 = k1 + k2 + k3 + 2√(k1k2 + k2k3 + k1k3)
(equation for inner circle)
k4 = k1 + k2 + (0) + 2√(k1k2 + k2(0) + k1(0))
(k3 = 0)
k4 = k1 + k2 + 2√(k1k2)
(simplify)
k4 = k1 + 2√(k1k2) + k2
(rearrange)
k4 = (√k1 + √k2)2
(factor)
√k4 = √k1 + √k2
(square root)
1/√r4 = 1/√r2 + 1/√r1
(substitute)
1/√r1 + 1/√r2 = 1/√r4
(rearrange)

The relationship between the three radii can also be written as r1 + r2 = r3, or a + b = c, which is fascinatingly similar to relationship of the three sides in a right angle, which according to Pythagorean’s Theorem is a2 + b2 = c2.  The only difference between the two equations is the exponents, which are opposite reciprocals.

Now that we have established the relationship of the radii for three kissing circles and a line, we can investigate some specialized cases that produce some unexpected sequences.  It should be noted that if two radii are the reciprocal of a square integer, then the third radius will be the reciprocal of the sum of those two integers squared.  In other words, if r1 = 1/m2 and r2 = 1/n2, then r3 = 1/(m + n)2.  This can be shown algebraically:

1/√r1 + 1/√r2 = 1/√r3
(radius relationship)
r1 + r2 = r3
(rewrite exponents)
(1/m2) + (1/n2) = r3
(substitute  r1 and r2)
m + n = r3
(simplify exponents)
(m + n)-2 = r3
(negative square both sides)
r3 = 1/(m + n)2
(rewrite exponent)

So if two kissing circles have a radii of 1, then r1 = 1 = 1/12 and m = 1 and r2 = 1 = 1/12 and n = 1, and the third kissing circle that is in between them will have a radius r3 = 1/(m + n)2 = 1/(1 + 1)2 = 1/22.  Continuing on in one direction, the circle between this new circle with radius 1/22 and the original circle 1/12 will have a radius of 1/(2 + 1)2 = 1/32, and the circle between this new circle with radius 1/32 and the original circle 1/12 will have a radius of 1/(3 + 1)2 = 1/42, and so on, so that sequence of new radii for each successive circle is r = 1/12, 1/22, 1/32, 1/42, … which are the square reciprocals of the counting numbers.

Square Reciprocals of Counting Numbers

Similarly, if we start with two kissing circles that have a radii of 1 but alternate directions left and right for every other circle, the sequence becomes r = 1/12, 1/12, 1/(1 + 1)2 =1/22, 1/(1 + 2)2 =1/32, 1/(2 + 3)2 =1/52, 1/(3 + 5)2 =1/82, … or r = 1/12, 1/12, 1/22, 1/32, 1/52, 1/82, … which are the square reciprocals of the Fibonacci numbers.
                                                                                                                                                  
Square Reciprocals of Fibonacci Numbers

One last specialized case of kissing circles and a line that should be mentioned are Ford circles, named after the twentieth century mathematician Lester R. Ford.  Starting with two circles that have radii of r = ½, place the center of one circle directly above the 0 on the number line and place the center of the other circle directly above the 1.  The new kissing circle formed in between these two circles will have a center directly above x = 1/2.  Furthermore, the circle between the circles x = 0 and x = 1/2 is x = 1/3, and the circle between the circles x = 1/2 and x = 1 is x = 2/3.  This process can be continued on indefinitely, where it is possible to place a new circle at the x-value of any irreducible fraction p/q.
                        
Ford Circles

Additionally, it can be proved that the kissing circle between any x = p1/q1 and x = p2/q2 is x = (p1 + p2)/ (q1 + q2).  For example, the kissing circle between the circles x = 1/2 and x = 2/3 is x = (1 + 2)/(2 + 3) = 3/5
                                      
To prove this, we must first recognize that the first two outer Ford circles both have x-values in the form of x = p/q and radii in the form of r = 1/2q2.  (For the left circle, p1 = 0 and q1 = 1, so that x = p1/q1 = 0 and r = 1/2q12 = 1/2.  For the right circle, p2 = 1 and q2 = 1, so that x = p2/q2 = 1 and r = 1/2q22 = 1/2.)  It can then be proved algebraically that any new kissing circle produced by two circles with both x-values in the form of x = p/q and radii in the form of r = 1/2q2 also have these properties, specifically x = (p1 + p2)/ (q1 + q2) and r = 1/2(q1 +q2)2.  First, to prove that r = 1/2(q1 +q2)2:

1/√r1 + 1/√r2 = 1/√r3
(radius relationship)
r1 + r2 = r3
(rewrite exponents)
(1/2q12) + (1/2q22) = r3
(subst r1 and r2)
2½q1 + 2½q2 = r3
(simplify exponents)
2½(q1 + q2) = r3
(factor)
2-1(q1 + q2)-2 = r3
(negative square both sides)
r3 = 1/2(q1 +q2)2
(rewrite exponent)

To prove that x = (p1 + p2)/ (q1 + q2), recall from above that x3 is the distance between the centers of the two outside kissing circles, which in this case is x3 = p2/q2p1/q1, and that x1 is the distance between the centers of an outside and inside kissing circle, which in this case we are trying to prove that x1 = (p1 + p2)/ (q1 + q2)p1/q1.  Also recall from above that x1 = 2√(r1r3), x3 = 2√(r1r2), and √r3 = √(r1r2)/(√r1 + √r2).  Therefore:

x1 = 2√(r1r3)
(given)
x1 = 2√r1√r3
(distribute)
x1 = 2√r1√(r1r2)/(√r1 + √r2)
(subst √r3 = √(r1r2)/(√r1 + √r2))
x1 = √r1x3/(√r1 + √r2)
(subst x3 = 2√(r1r2))
x1 = √(1/2q12)(p2/q2p1/q1) /
(√(1/2q12) + √(1/2q22))
(subst r1 = 1/2q12, r2 = 1/2q22, and x3 = p2/q2p1/q1)
x1 = 1/q1(p2/q2p1/q1) /
(1/q1 + 1/q2)
(simplify)
x1 = (q1p2 – q2p1)/q1(q2 + q1)
(multiply by q1q1q2/q1q1q2)
x1 = (q1p2 + q1p1 – q1p1 – q2p1)/q1(q2 + q1)
(add and subtract q1p1)
x1 = q1(p2 + p1) – p1(q1 – q2)/q1(q2 + q1)
(factor)
x1 = q1(p2 + p1)/q1(q2 + q1)
p1(q1 – q2)/q1(q2 + q1)
(distribute denominator)
x1 = (p2 + p1)/(q2 + q1) p1/q1
(simplify)

In conclusion, the relationship between the three kissing circles and a line is 1/√r1 + 1/√r2 = 1/√r3.  This can be rewritten as r1 + r2 = r3, which is fascinatingly similar to Pythagorean’s Theorem.  Furthermore, it can be shown that the radii of a series of kissing circles under two kissing unit circles and a line in one direction is a sequence of square reciprocals of the counting numbers, and in alternating directions is a sequence of square reciprocals of the Fibonacci numbers.  Finally, Ford circles are kissing circles in which it is possible to place a new circle at the x-value of any irreducible fraction p/q, and any successive kissing circle between x = p1/q1 and x = p2/q2 will have a new x-value of x = (p1 + p2)/ (q1 + q2)