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2022-08-26
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Math Reflections
One of the fascinating facets of
mathematics is the huge number of
everyday occurrences that provide
proof for the fundamentals underlying
the structure of mathematics.
In this instance, we're going to use
the random falling of hail stones to
derive an approximation of the value
of pi.
First, let's get a grasp of the lay
of the land. Good old Sven Svensen has
planted a huge square plot with wheat.
The area of Sven's acreage is
4,000,000 square meters, which happens
to be 4 square kilometers.
Now let's imagine a circle inscribed
within this square. There's no good
reason for Sven to imagine such a
circle, but we need the circle for
plot (no pun intended) development.
Let's stretch our imagination even
further and divide Sven's field into
quadrants, each having an area of
1 square kilometer.
We can now use the definitions of
Euclidean geometry to derive some
useful information about our imaginary
subdivisions.
We can determine that the length of
a side of our imaginary quadrant is
1 kilometer. (Length of a side of a
square is equal to the square root of
the area of the square. The area is
1 sq. km, so the side is 1 km.)
We also know by definition that the
radius of an inscribed circle is equal
to the length of a side of a quadrant
of the square circumscribing the
circle. Isn't this fun?
So far we haven't found out anything
about pi and, since we know that wheat
has very little to do with pie, we
seem to be out in left field instead
of firmly rooted in Sven's wheat
field. In fact, Sven's field is about
to be devastated by a hail storm.
But every cloud has a silver lining
and the silver lining here is that we
can use Sven's misfortune to confirm
mathematicians' approximations of the
value of pi.
How? Obviously not by the very
empirical approach of counting the
hail stones. For one thing, that could
be painful. For another, some of the
hail stones would melt before we got
them tabulated (remember, Sven has a
big field here).
Instead, we'll use probability
theorems to construct a method by
which we can determine certain useful
information.
Instead of focussing on Sven's
entire field, let's direct our
attention at one quadrant and its
quarter of a circle.
Presumably, hail does not fall at
the direction of some Greater
Intelligence, so we can assume that it
is relatively uniform in its
distribution over the land on which it
falls.
We can understand that every hail
stone that lands in the quarter circle
also lands in the key quadrant. But
the converse is not true, some of the
hail that falls in the quadrant will
fall outside of the inscribed quarter
circle.
And, because Sven is such a model of
an excellent farmer, we can assume
that he's planted each part of his
field equally as densely as any other
part.
What we intend to measure is the
number of wheat stalks damaged by the
hail in the quadrant of our interest.
Not only are we interested in the
total number of wheat stalks damaged,
we're also interested in how many of
those stalks fell within the inscribed
circle.
Probability theory tells us that if
we can measure the damage accurately,
we can derive some good stuff ... like
an approximation of the value of pi.
Let's leave Sven, mourning over the
destruction of his wheat field, to
contemplate the wonders of probability
theory as they relate to the single
quadrant of Sven's field that we are
interested in.
The science/art of probability is
not so arcane as to sound like Greek.
In fact, we can intuit the first law
that we're interested in. Let's derive
a rule of probability for determining
how many of the wheat stalks that are
damaged in this quadrant are inside
the quarter circle as well.
If we simplify the problem by
dividing the quadrant into two equal
halves, we can restate the problem as
"How many of the wheat stalks that are
damaged in this quadrant will be in
the North half of this quadrant?"
A good guess (and a correct one)
would be half of them, because we're
dealing with half of the area.
We can generalize this guess to say
that the probability of a wheat stalk
in a given portion of a field being
damaged is represented by the ratio of
the area of the portion to the area of
the whole field.
For the sake of simplification,
let's call the area circumscribed by
the quadrant Q (for quadrant) and
let's call the area of the quarter
circle that exists inside the
quadrant C (for circle).
In formula-ese, we can now express
our discovered generality in the
following terms:
Area of C
Prob (w in C) = ---------
Area of Q
As more and more wheat stalks are
randomly damaged by the hail, this
formula can be restated as:
N(C)
Prob (w in C) = ----
N(Q)
where N represents the number of
damaged wheat stalks observed in the
area.
This substitution can also be
intuitively derived. Think of the
quadrant divided in half again. It's
entirely believable that if 100 wheat
stalks are damaged in the whole
quadrant then 50 of them will be found
in the north half. The above formula
just generalizes this rule.
So much for theory, let's put it to
practice. Remember that the side of
the quadrant is equal to the radius of
the inscribed circle and that exactly
one-quarter of the circle lies within
the quadrant.
Therefore, substituting the real
values of area into the first equation
gives us the following:
2
Area of C (1/4) PI R
--------- = ----------- = (1/4) PI
2
Area of Q R
Substituting this result into the
second equation results in this
formula:
> N(C)
PI = 4 ----
N(Q)
Does the light begin to dawn?
We left Sven waiting for the
insurance adjuster to come and inspect
the damage to his wheat.
If each and every damaged wheat
stalk were counted and the resulting
values inserted into our formula for
pi, were should get a reasonable
approximation of 3.14159... (within .1
of accuracy).
We don't intend to go out to Sven's
and count wheat stalks. That's too
much like work. Instead, we've devised
a simulation that pretty much depicts
the random destruction of the wheat by
the hail.
The way it works, each dot
represents a damaged wheat stalk. The
program randomly places these dots,
thus simulating the dropping of hail
on the wheat. At all times, the
program is counting the number of dots
placed and their relationship to the
quadrant and the square.
Run it from here and see two things:
the total destruction of Sven's wheat
and an interesting approximation of
the value of pi. To see the current
approximation of pi, press the space
bar. Then to return to the
destruction, press it again. To exit
the program, press the 'Q' key.
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