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The vertices of the square are given as complex numbers: $2+i$, $a$, $7-3i$, and $b$. Since these points form a square, the diagonals are perpendicular, and the midpoint of one diagonal is the center of the square. Let's use this information to solve for $a$ and $b$.

The midpoint of the diagonal connecting $2+i$ and $7-3i$ can be found as:

$$\text{Midpoint} = \frac{(2+i) + (7-3i)}{2} = \frac{9 - 2i}{2} = \frac{9}{2} - i.$$

This midpoint should be equal to the midpoint of the diagonal connecting $a$ and $b$:

$$\frac{a + b}{2} = \frac{9}{2} - i.$$

From this equation, we can solve for $b$:

$$b = 9 - 2a - 2i.$$

Now, since the vertices are arranged in counterclockwise order, $2+i$ and $a$ are adjacent vertices of the square. The side connecting these two vertices has the same length as the side connecting $7-3i$ and $b$. This gives us the following relationship:

$$\|2+i - a\| = \|7-3i - b\|.$$

Substituting the expressions for $a$ and $b$, we get:

$$\|2+i - a\| = \|7-3i - (9 - 2a - 2i)\|.$$

Simplify the expressions:

$$\|2+i - a\| = \|2a - 2i\|.$$

Since the lengths of the sides are equal, the magnitudes of the differences are equal:

$$\|2+i - a\| = \|2a - 2i\|.$$

Square both sides of the equation:

$$(2+i - a)(\overline{2+i - a}) = (2a - 2i)(\overline{2a - 2i}).$$

Expand both sides:

$$(2+i - a)(2-i - \bar{a}) = (2a - 2i)(2a + 2i).$$

Simplify further:

$$(4 - a - ai + 2i - 2i + i^2 + \bar{a} - \bar{a}i + a\bar{a}) = (4a^2 - 4i^2).$$

Since $i^2 = -1$:

$$(4 - 2ai + a^2 + \bar{a} - a\bar{a}) = (4a^2 + 4).$$

Rearrange the terms:

$$(4 + 4a^2) - 2ai + a^2 + \bar{a} - a\bar{a} = 4a^2 + 4.$$

Simplify and collect the real and imaginary terms:

$$5 - 3a^2 - 2ai + \bar{a} - a\bar{a} = 0.$$

Since $a$ is a real number, $\bar{a} = a$:

$$5 - 3a^2 - 2ai + a - a^2 = 0.$$

Combine like terms:

$$5 - 4a^2 - 2ai + a = 0.$$

Solve for $a$:

$$5 - 4a^2 - ai = -a.$$

$$5 - 4a^2 = -ai + a.$$

$$5 - 4a^2 = a(1 - i).$$

$$a = \frac{5 - 4a^2}{1 - i}.$$

Multiply the numerator and denominator by the conjugate of the denominator to rationalize it:

$$a = \frac{(5 - 4a^2)(1 + i)}{(1 - i)(1 + i)}.$$

$$a = \frac{5 + 5i - 4a^2 - 4a^2i}{2}.$$

$$a = \frac{5 - 8a^2 + 5i - 4a^2i}{2}.$$

Equating the real and imaginary parts separately:

$$a = \frac{5 - 8a^2}{2} \quad \text{(real part)},$$
$$a = \frac{5 - 4a^2}{2} \quad \text{(imaginary part)}.$$

Solve each equation for $a$:

$$5 - 8a^2 = 2a.$$
$$5 - 4a^2 = 2a.$$

For the first equation:

$$8a^2 + 2a - 5 = 0.$$

Using the quadratic formula:

$$a = \frac{-2 \pm \sqrt{2^2 - 4 \cdot 8 \cdot (-5)}}{2 \cdot 8}.$$

Simplify under the square root:

$$a = \frac{-2 \pm \sqrt{4 + 160}}{16}.$$

$$a = \frac{-2 \pm \sqrt{164}}{16}.$$

Since $164$ is not a perfect square, we'll leave it as is:

$$a = \frac{-2 \pm \sqrt{41} \sqrt{4}}{16}.$$

$$a = \frac{-1 \pm \sqrt{41}}{8}.$$

For the second equation:

$$4a^2 + 2a - 5 = 0.$$

Using the quadratic formula again:

$$a = \frac{-2 \pm \sqrt{2^2 - 4 \cdot 4 \cdot (-5)}}{2 \cdot 4}.$$

Simplify under the square root:

$$a = \frac{-2 \pm \sqrt{4 + 80}}{8}.$$

$$a = \frac{-2 \pm \sqrt{84}}{8}.$$

Since $84$ is not a perfect square, we'll leave it as is:

$$a = \frac{-2 \pm \sqrt{4} \sqrt{21}}{8}.$$

$$a = \frac{-1 \pm 2\sqrt{21}}{8}.$$

So, the possible values for $a$ are:

$$a = \frac{-1 + \sqrt{41}}{8}, \quad a = \frac{-1 - \sqrt{41}}{8}, \quad a = \frac{-1 + 2\sqrt{21}}{8}, \quad a = \frac{-1 - 2\sqrt{21}}{8}.$$

Now, we have the expression $b/a$. Let's find the possible values of $b$ for each of the values of $a$:

For $a = \frac{-1 + \sqrt{41}}{8}$:

$$b = 9 - 2a - 2i = 9 - 2\left(\frac{-1 + \sqrt{41}}{8}\right) - 2i.$$

Simplify:

$$b = \frac{9 + \sqrt{41}}{4} - 2i.$$

For $a = \frac{-1 - \sqrt{41}}{8}$:

$$b = 9 - 2a - 2i = 9 - 2\left(\frac{-1 - \sqrt{41}}{8}\right) - 2i.$$

Simplify:

$$b = \frac{9 - \sqrt{41}}{4} - 2i.$$

For $a = \frac{-1 + 2\sqrt{21}}{8}$:

$$b = 9 - 2a - 2i = 9 - 2\left(\frac{-1 + 2\sqrt{21}}{8}\right) - 2i.$$

Simplify:

$$b = \frac{9 + 2\sqrt{21}}{4} - 2i.$$

For $a = \frac{-1 - 2\sqrt{21}}{8}$:

$$b = 9 - 2a - 2i = 9 - 2\left(\frac{-1 - 2\sqrt{21}}{8}\right) - 2i.$$

Simplify:

$$b = \frac{9 - 2\sqrt{21}}{4} - 2i.$$

So, the possible values of $b/a$ for each value of $a$ are:

For $a = \frac{-1 + \sqrt{41}}{8}$:

$$\frac{b}{a} = \frac{\frac{9 + \sqrt{41}}{4} - 2i}{\frac{-1 + \sqrt{41}}{8}} = \frac{8(9 + \sqrt{41}) - 8(2i)}{4(-1 + \sqrt{41})}.$$

Simplify:

$$\frac{b}{a} = \frac{72 + 8\sqrt{41} - 16i}{-4 + 4\sqrt{41}}.$$

For $a = \frac{-1 - \sqrt{41}}{8}$:

$$\frac{b}{a} = \frac{\frac{9 - \sqrt{41}}{4} - 2i}{\frac{-1 - \sqrt{41}}{8}} = \frac{8(9 - \sqrt{41}) - 8(2i)}{4(-1 - \sqrt{41})}.$$

Simplify:

$$\frac{b}{a} = \frac{72 - 8\sqrt{41} - 16i}{-4 - 4\sqrt{41}}.$$

For $a = \frac{-1 + 2\sqrt{21}}{8}$:

$$\frac{b}{a} = \frac{\frac{9 + 2\sqrt{21}}{4} - 2i}{\frac{-1 + 2\sqrt{21}}{8}} = \frac{8(9 + 2\sqrt{21}) - 8(2i)}{4(-1 + 2\sqrt{21})}.$$

Simplify:

$$\frac{b}{a} = \frac{72 + 16\sqrt{21} - 16i}{-4 + 8\sqrt{21}}.$$

For $a = \frac{-1 - 2\sqrt{21}}{8}$:

\[\frac{b}{a} = \frac{\frac{9 - 2\sqrt{21}}{4} - 2i}{\frac{-1 - 2\sqrt{21}}{8}} = \frac{8(9 - 2\sqrt

{21}) - 8(2i)}{4(-1 - 2\sqrt{21})}.\]

Simplify:

$$\frac{b}{a} = \frac{72 - 16\sqrt{21} - 16i}{-4 - 8\sqrt{21}}.$$

In conclusion, the possible values of $b/a$ for the given values of $a$ are:

$$\frac{b}{a} = \frac{72 + 8\sqrt{41} - 16i}{-4 + 4\sqrt{41}}, \quad \frac{b}{a} = \frac{72 - 8\sqrt{41} - 16i}{-4 - 4\sqrt{41}}, \quad \frac{b}{a} = \frac{72 + 16\sqrt{21} - 16i}{-4 + 8\sqrt{21}}, \quad \frac{b}{a} = \frac{72 - 16\sqrt{21} - 16i}{-4 - 8\sqrt{21}}.$$