THE MARVELOUS SQUARE ROOT OF TWO
So many of the relationships in this design I’ve drawn involve the marvelous square root of 2.
Full-size image
Most interesting I think is the fact that the ratio of the diameters of the circles, green circle to red, and red to white, is a constant. It’s also interesting the way the values of the diameters add up, and the way they relate to the sides of the squares. More on this later below. I started a little journey here with my high school math. Won’t you join me?
Dimensions of a triangle given
In my graphic below, a diamond shape composed of four isosceles right triangles (45-90-45 degrees) forms a perfect square, set on end. Let a = b = 1, where “a” is the altitude of a triangle and “b” is the base. Using Pythagoras’ formula, we find that the third and longest side of the triangle, “c,” (the hypotenuse) is equal to √(a2 + b2), that is, √2.
Full-size image with colors labeled (click here, then on the image)
Radius and diameter of a green circle
Next we find “r,” the radius of the green circle inscribed in the triangle, and also as we see later, the radius of the corner green circles. See graphic 795611, where my formula for this is given as r = ab / (a + b + c) = 1 / (1 + 1 + √2) = 1 / (2 + √2) = (1 / (2 + √2)) x ((2 – √2) / (2 – √2)) = (2 – √2) / (4 – 2√2 + 2√2 – 2) = (2 – √2) / 2. The diameter of a green circle is thus (2 – √2). The diameter of the green circle can be expressed simply as a + b – c.
Radius and diameter of a red circle
The radius of a red circle equals a – r or b – r, that is, 1 – ((2 – √2) / 2) = (2 / 2) – ((2 – √2) / 2) = √2 / 2. The diameter of a red circle is thus √2.
Radius and diameter of the white circle
The radius of the enclosing or white circle equals a or b plus the radius of a red circle, that is, 1 + (√2 / 2) = (2 / 2) + (√2 / 2) = (2 + √2) / 2. The diameter of the white circle is thus 2 + √2.
Circles are tangential
Two equal red circles, centered at diamond vertices, meet and are tangential at the midpoint of c, the hypotenuse. It is clear that the diameter of a red circle, √2, equals c, also √2. The red circles are tangential to the green circle (darkened) at the center of the design. An inspection of the math confirms this: radius of red circle (√2 / 2) plus radius of green circle ((2 – √2) / 2) equals (√2 + (2 – √2)) / 2 = 2 / 2 = 1. A similar inspection using the yellow lines parallel to a and b respectively, reveals that the corner green circle has a radius of (2 – √2) / 2 and is tangential to its neighboring red circles. Thus the inscribed green circles and the corner green circles and center green circle are all equal. The corner green circle is tangential to the white circle in that the sum of “d” (equal to and perpendicular to c) which is √2, plus the radius of a green circle ((2 – √2) / 2) equals (√2 x (2 / 2)) + ((2 – √2) / 2) = ((2 √2) / 2) + ((2 – √2) / 2) = ((2 √2) + (2 – √2)) / 2 = the radius of the white circle (2 + √2) / 2.
Ratios of diameters
The ratio of the diameters of the circles, green circle to red, and red to white, is a constant, (√2 – 1) to 1, a value which also happens to equal tan 22.5 degrees. Here are the ratios calculated: Green diameter to red diameter = (2 – √2) / √2 = ((2 – √2) / √2) x (√2 / √2) = (2√2 – 2) / 2 = √2 – 1. Red diameter to white diameter = √2 / (2 + √2) = (√2 / (2 + √2)) x ((2 – √2) / (2 – √2)) = (2√2 – 2) / (4 – 2√2 + 2√2 – 2) = (2√2 – 2) / 2 = √2 – 1. Of course the ratios of radii will follow the same pattern and have the same constant.
Radius of inscribed circle is confirmed
We can confirm the radius of the circle inscribed in the isosceles right triangle by constructing a line “h” from the left-most vertex (45 degree angle) through the center of the circle. If the circle is inscribed and has the radius as calculated above, then this line will bisect the angle yielding an angle of 22.5 degrees. Setting up an equation, tan of angle x = opposite / adjacent = green radius / red radius = ((2 – √2) / 2) / (√2 / 2) = (2 – √2) / √2 = ((2 – √2) / √2) x (√2 / √2) = (2√2 – 2) / 2 = √2 – 1. A glance at a trig chart confirms this is indeed the tan of 22.5 degrees (0.4142).
Sum of two diameters is 2
At some point I noticed that the sum of the diameter of a green circle plus the diameter of a red circle equals 2, when a = 1, as follows: (2 – √2) + √2 = 2.
Sum of two radii is 1
Naturally, the sum of the radius of a green circle plus the radius of a red circle equals 1, when a = 1, as follows: ((2 – √2) / 2) + (√2 / 2) = ((2 – √2) + √2) / 2 = 2 / 2 = 1
Product of two diameters is 2
Later, someone pointed out to me that the product of the diameter of a green circle and the diameter of the white circle equals 2, when a = 1, as follows: (2 – √2) x (2 + √2) = (4 + 2√2 – 2√2 – 2) = 2. She got me started down this avenue more than a year ago by saying, “Did you realize a 45-90-45 degree triangle can have sides of 1 – 1 – √2?”
Diamond measures 2
I don’t know if it is worth mentioning that the height and the width of the diamond are also 2, when a = 1; (1 + 1 = 2). In other words, the diamond-square with sides of √2, has diagonals each equal 2.
Side of next square is 2
The sides of the next larger square equal 2. This square is indicated by yellow lines in the graphic.
This design set to the next is ratio √2 to 1
Just as this design has circles associated with a square with sides of √2, there is a “similar” design (geometrically similar) associated with a square with sides of 1, formed by connecting in succession the midpoints of the previous square with sides of √2. Just as the sides of these two squares are in a ratio of √2 to 1, the circle designs associated with these two squares are also in a ratio of √2 to 1. Thus, the diameters of the circles in the first design (2 – √2, √2, and 2 + √2), when divided by √2, yield the following values for the diameters in the second design: √2 – 1, 1, and √2 + 1. These latter circles are yellow, blue, and lavender in my drawings above and below.
Full-size image
Diameter is the value of the constant
Notice that the diameter of the small circle (yellow) associated with the square of side 1, is √2 – 1, which equals the value of the constant (ratios of small to medium, medium to large diameters among the circles of a given square), and this value is also equal to the tan 22.5 degrees.
Diameter is the value of the reciprocal of the constant
The diameter of the large circle (lavender) in this design set, √2 + 1, is the reciprocal of the constant √2 – 1 (meaning 1 / (√2 – 1)), and thus represents the ratios of large to medium, medium to small diameters among the circles of a given square – just the reverse order as paragraph above. Therefore, the reciprocal √2 + 1 is also a constant.
Diameter is quotient of identical values
The diameter of the medium circle (blue) in this design set, 1, is the quotient of the diameter of the small circle (yellow) √2 – 1 and identical value, the constant √2 – 1. Of course, any number divided by itself is 1.
Diameter is quotient of other identical values
Another way of looking at this is that the diameter of the medium circle (blue) in this design set, 1, is the quotient of the diameter of the large (lavender) circle, √2 + 1 and the identical value, the reciprocal constant √2 + 1. Of course, any number divided by itself is 1.
Product of diameters is 1
The product of the diameter of the large (lavender) circle, √2 + 1, and the diameter of the small (yellow) circle, √2 – 1, equals 1, the diameter of the medium (blue) circle. Of course, any number multiplied by its reciprocal is 1. The calculation for this is (√2 + 1) x (√2 – 1) = (2 – √2 + √2 – 1) = 1.
Diameters and the sides of the squares
As shown in the graphic above, we have three squares with sides of 2, √2, and 1, each square nestled within the previous. Notice that the values for the circle diameters in each design set are equal to either the difference of the sides of the squares, the sides of the squares, or the sum of the sides of the squares. Each small circle in a design set is equal to the difference of the sides of two squares (2 – √2 and √2 – 1). Each medium circle in a design set is equal to a side of a square (√2 and 1). Each large circle in a design set is equal to the sum of the sides of two squares (2 + √2 and √2 + 1).
Area of design set is double that within
The area of a square is double the square within it. Area of a square is a side squared, so for squares with sides of 2, √2, and 1, the areas are 4, 2, and 1; each double the square within. Likewise, the areas of circles in a given design set are double the areas of circles in the next design set within it. The area of a circle is Π x (radius)2, so the areas of blue and red circles are 1/4 Π and 1/2 Π respectively, latter twice the former. While dimensions differ by a factor of √2 between design sets, the areas differ by a factor of 2.
Design dimensions double
With every second iteration of the design (factor √2 x factor √2) the design dimensions double.
A circle with a diameter of 2
If I drew the next larger design set, the medium circle in that set (red-similar) would have a diameter of 2. This diameter would be the diameter of the red circle (√2) multiplied by the enlargement factor √2, or (√2)2. A diameter of 2 can also be thought of as the product of the red diameter times the red diameter. A diameter of 2 can also be thought of as the sum of two blue circle diameters (1 + 1 = 2).
Intersection of three circles
The center of a small (yellow) inscribed circle is at the intersection of a red circle and the center green circle. The calculation for this: by inspection, the distance from the center of the red circle to the boundary of the square with side of 1 is 1/2. Add to this the radius of the yellow circle, √2 – 1. The sum should be √2/2, the radius of the red circle, and it is: (1/2) + ((√2 – 1) / 2) = (1 + (√2 – 1)) / 2 = √2 / 2.
Concentric circles
The corner yellow circles are concentric with the red circles.
Diameters equal to trigonometric values
Other nice relationships in this design: both sin and cos of 45 degrees equal 1 / √2, or put another way, 1 / √2 = (1 / √2) x (√2 / √2) = √2 / 2, the radius of a red circle. The tan of 45 degrees is 1, the diameter of a blue circle. As mentioned earlier, the diameter of a yellow circle, √2 – 1, is the tan of 22.5 degrees.
See how the diameters add up
I decided to get a bit more methodical in looking at sums and so made the following table which shows the diameter values of two design sets, designated greater and lesser.
Diameter values of two design sets
|
Relative diameter size
|
Left
|
Small
|
Medium
(center value)
|
Large
|
Right
|
Greater set
|
3√2 – 4
|
2 – √2
green
|
√2
red
|
2 + √2
white
|
3√2 + 4
|
Lesser set
|
3 – 2√2
|
√2 – 1
yellow
|
1
blue
|
√2 + 1
lavender
|
3 + 2√2
|
Rule 1: The sum of any two consecutive diameters in a column equals the value in the next column for the lesser design set. For example:
>> green (2 – √2) + yellow (√2 – 1) = blue (1), and
>> red (√2) + blue (1) = lavender (√2 + 1)
Rule 2: The sum of any two consecutive diameters in a row equals the value in the column above the right-side figure. For example:
>> yellow (√2 – 1) + blue (1) = red (√2), and
>> blue (1) + lavender (√2 + 1) = white (2 + √2)
Rule 3: Notice that values on either side of the “center value” make up matched pairs that differ only in sign. In the lesser design set, the product of each matched pair is 1. In the greater design set, the product of each matched pair is 2. For example:
>> (√2 + 1 ) x (√2 – 1) = (2 – √2 + √2 – 1) = 1.
>> (3√2 – 4) x (3√2 + 4) = (18 + 12√2 – 12√2 – 16) = 2
I got out pencil and paper and my guess is that this progression continues with products developing in succession, each a factor of two greater than the previous: 1, 2, 4, 8, 16 . . . etc.
One more sum and product
Both the sum and the product of red and white diameters equal the value of the large circle (2√2 + 2) in the next greater design set.
I suppose there could be even more interesting relationships in these designs, but that’s enough for now.
-July 19, 2010-
-updated August 1, 2010-
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My related drawings:
794206 Each circle 2X diameter of two before, factor square root 2 over previous, 2X area of previous
795805 Each square 2X width of two before, factor square root 2 over previous, 2X area of previous
795811 Wreath circle diameter is the difference of the sides of the squares (square root 2 – 1) (circles in triangles oriented differently from above to form wreaths in this and next two drawings)
795811b Graphic for diameter is the difference of the sides of the squares (square root 2 – 1)
795812 Outer wreath circle diameter is the difference of the sides of the squares (2 – square root 2), ratio diameter to diameter is square root 2
796207 Five equal circles
796219 Design set added twice dimensions of first set