Chapter 37. Areas of the Flag
From Robert Banks, Slicing Pizzas, Racing Turtles, and
Further Adventures in Applied Mathematics
This project is simple: it does not use loops, if-statements or any
data structures. This exercise focuses on expressions and assignment
Facts about the American flag. The standard dimension is the
hoist). This is the basic unit that all others are multiplied by. A flag
that is 30 inches wide will simply have 30 inches multiplied by all of
the other measurements.
- Width (or
- Length (or
) Lf=1.9 × Wf
- Width of union (or
) Wu=7/13 ×
- Length of union Lu=0.76 × Wf
- Radius of a star R=0.0308 × Wf
These are other facts; they are counts, not measurements.
- Number of red stripes Sr=7
- Number of white stripes Sw=6
- Number white stars Ns=50
- Width of a stripe Ws=1/(Sr+Sw) × Wf
Red Area. There are 4 red stripes which abut the blue field and 3 red
stripes run the full length of the flag.
We can compute the area of the red, since it is 4 short rectangles
and 3 long rectangles. The short rectangle areas are the width of a stripe
(Ws) times the whole length (length of the fly, Lf) less the width of the
blue union (Wu). The long rectangle areas are simply the width of the
stripe (Ws) times the length of the fly (Lf).
Red = 4×Ws×(Lf-Lu) + 3×Ws×(Lf).
White Area. There are 3 white stripes which abut the blue field, 3 whie
stripes run the full length of the flag, plus there are the 50
We can compute the basic area of the white using a similar analysis
as area of the red, and adding in the areas of the stars,
. We'll return to the area of the stars,
White = 3×Ws×(Lf-Lu) + 3×Ws×(Lf) + 50S.
Blue Area. The blue area is the area of the union, less the area of the
Blue = (Lu-Wu) - 50S.
Area of the Stars. A 5-sided star (pentagram) can be analyzed as 5 kites of 2
triangles. The area of each kite,
, is computed as
Figure 37.1. Kite and Star
The inner angles of all five kites fill the inside of the pentagram,
and the angles must sum to 360°, therefore each inner angle is 72°. Angles
of a triangle sum to 180°, therefore the lower triangle has two side
angles of (180°-72°)/2 = 54°.
We see some straight lines that contain an outer triangle and two
inner triangles. We know the inner triangles add to 108°, a straight line
is 180° so the outer triangle has two 72° corners and a 36° peak. The area
of the two triangles can be computed as
Equation 37.1. Area of a triangle, version 1
Equation 37.2. Area of a triangle, version 2
Note that the math library
cos functions operate in radians, not degrees. The
conversion rule is π radians = 180 degrees. Therefore, we often see
) to convert
a degrees to radians.
The Golden Ratio is
The total area of a star is
Given a specific width of a flag (in feet), we can compute the
actual areas of each color.
Check Your Results. Blue is 18.73% of the total area.