I read the other answers already provided for this particular problem, and I could not make sense of them, so I will post my own approach to this problem. Before we start with the nitty-gritty details of this problem, we should first determine what possible values of A exist in base 63. In the base 63 system, the smallest possible digit is the digit representing 0 in base 10, and the largest possible digit represents 62 in base 10. Therefore, we conclude that \(0 \leq A \leq 62\). This will become important when we generate values of A.
First, I will expand the base 63 number into its more familiar base 10 form by expansion. However, since we only care about the unit's digit of this particular number, I will simplify as much as possible to make the computation easier.
\(A7894321_{63} = 1 \times 63^0 + 2 \times 63^1 + 3 \times 63^2 + 4 \times 63^3 + 9 \times 63^4 + 8 \times 63^5 + 7 \times 63^6 + A \times 63^7\)
The first observation I made is that the unit's digit of \(63^p\) and \(3^p\) for nonnegative integer powers p have the same unit's digit. This occurs because \(63^p \pmod{10} \equiv 3^p \pmod{10}\). This means I can simplify the nasty expansion to the following. In order to ensure that I am writing mathematically, I will define a new function \(U(x)\) which returns the unit's digit of an integer x.
\(U(A7894321_{63}) = 1 \times 3^0 + 2 \times 3^1 + 3 \times 3^2 + 4 \times 3^3 + 9 \times 3^4 + 8 \times 3^5 + 7 \times 3^6 + A \times 3^7\)
I began exploring the powers of 3, and I immediately noticed a pattern among the powers of 3 and their units digit.
p | 0 | 1 | 2 | 3 | 4 | 5 | 6 | ... |
3p | 1 | 3 | 9 | 27 | 81 | 243 | 729 | ... |
U(3p) | 1 | 3 | 9 | 7 | 1 | 3 | 9 | ... |
Observing the first few values of \(3^p\) should convince yourself that a never-ending pattern exists where 3 raised to the power of 0, 1, 2, and 3 have unique unit's digits, but the pattern repeats thereafter. This essentially means that we can simplify the powers further by applying the rule that \(U\left(3^p\right) = U\left(3^{p \pmod{4}}\right)\)
\(U(A7894321) = 1 \times 3^0 + 2 \times 3^1 + 3 \times 3^2 + 4 \times 3^3 + 9 \times 3^0 + 8 \times 3^1 + 7 \times 3^2 + A \times 3^3\)
Given our observations from the previous investigation of the powers of 3, we can reference that table to determine the unit's digit of the powers of 3.
\(U(A7894321) = 1 \times 1 + 2 \times 3 + 3 \times 9 + 4 \times 7 + 9 \times 1 + 8 \times 3 + 7 \times 9 + A \times 7\)
We can do these multiplications without a calculator unlike before where the calculation would have been unfeasible.
\(U(A7894321) = 1 + 6 +27 + 28 + 9 + 24 + 63 + 7A\)
We could do the addition now, but we can reduce the two-digit numbers to its final digit and do the addition because we only care about the unit's digit.
\(\begin{align*} U(A7894321) &= 1 + 6 + 7 + 8 + 9 + 4 + 3 + 7A \\ &= 38 + 7A \\ &= 8 + 7A \end{align*} \)
We finally have a nice and compact representation of the unit's digit. Of course, we want the unit's digit of this base 63 number to be zero. In order to make \(U(8 + 7A) = 0\). The first part of the addition has a unit's digit of 8. In order to make the sum have a final digit of 0, it would be required that \(U(7A) = 2\).
I will once again try to generate a table of values and observe a pattern
A | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | ... |
7A | 0 | 7 | 14 | 21 | 28 | 35 | 42 | 49 | 56 | 63 | 70 | 77 | 84 | ... |
U(7A) | 0 | 7 | 4 | 1 | 8 | 5 | 2 | 9 | 6 | 3 | 0 | 7 | 4 | ... |
Based on these observations, \(U(7A) = 2\) when \(U(A) = 6\). In other words, the unit's digit of A needs to be 6. Now, all we need to do is list the values of A with a unit's digit of 6. Also, recall from the beginning that we determined that \(0 \leq A \leq 62\).
This means that \(A = \{6, 16, 26, 36, 46, 56\}\). All values in this set are possible values for A.
Use the pythagorean theorem to find that hypotenuse BC = 6*sqrt(2)
Or you can use the 45-45-90 triangle template.
Let D = midpoint of B and C
It turns out that any right triangle will have its circumcenter at the midpoint of the hypotenuse.
This means we can draw a circle centered at D, and radius 3*sqrt(2).
This circumcircle passes through A, B, and C.
The distance from A to D is 3*sqrt(2), aka the radius of this new circle.
Therefore, segment AD = 3*sqrt(2)
Use this info to determine the radius of each small arc.
To get the distance along a circle's edge, you'll need this formula
arc length = (angle in radians)*r
45 degrees = pi/4 radians
90 degrees = pi/2 radians
I'll let the student finish up from here.