Complex Number
1. **State the problem:** Given the complex number equation $\frac{\overline{Z}}{Z} = \frac{3}{5} + \frac{1}{5}i$ and the condition $Z + \overline{Z} = 4$, find the complex number $Z$.
2. **Recall important formulas and rules:**
- For a complex number $Z = x + yi$, its conjugate is $\overline{Z} = x - yi$.
- The sum $Z + \overline{Z} = 2x$ (twice the real part).
- The quotient $\frac{\overline{Z}}{Z}$ can be simplified using $Z = re^{i\theta}$, so $\frac{\overline{Z}}{Z} = e^{-2i\theta}$, which lies on the unit circle.
3. **Use the given condition $Z + \overline{Z} = 4$:**
$$Z + \overline{Z} = 2x = 4 \implies x = 2$$
4. **Express $Z$ and $\overline{Z}$:**
$$Z = 2 + yi, \quad \overline{Z} = 2 - yi$$
5. **Use the quotient equation:**
$$\frac{\overline{Z}}{Z} = \frac{2 - yi}{2 + yi} = \frac{(2 - yi)(2 - yi)}{(2 + yi)(2 - yi)} = \frac{(2 - yi)^2}{4 + y^2}$$
6. **Calculate numerator:**
$$(2 - yi)^2 = 4 - 4yi + y^2 i^2 = 4 - 4yi - y^2$$
7. **So:**
$$\frac{\overline{Z}}{Z} = \frac{4 - y^2 - 4yi}{4 + y^2} = \frac{4 - y^2}{4 + y^2} - \frac{4y}{4 + y^2}i$$
8. **Equate real and imaginary parts to given values:**
$$\frac{4 - y^2}{4 + y^2} = \frac{3}{5}, \quad -\frac{4y}{4 + y^2} = \frac{1}{5}$$
9. **Solve the real part equation:**
$$5(4 - y^2) = 3(4 + y^2) \implies 20 - 5y^2 = 12 + 3y^2 \implies 8 = 8y^2 \implies y^2 = 1$$
10. **Solve the imaginary part equation:**
$$-\frac{4y}{4 + y^2} = \frac{1}{5} \implies -\frac{4y}{4 + 1} = \frac{1}{5} \implies -\frac{4y}{5} = \frac{1}{5} \implies -4y = 1 \implies y = -\frac{1}{4}$$
11. **Check consistency:** From step 9, $y^2=1$ so $y=\pm 1$, but from step 10, $y = -\frac{1}{4}$. This is a contradiction, so re-examine step 5.
12. **Alternative approach:** Multiply numerator and denominator by conjugate of denominator:
$$\frac{2 - yi}{2 + yi} = \frac{(2 - yi)(2 - yi)}{(2 + yi)(2 - yi)} = \frac{4 - 4yi + y^2 i^2}{4 + y^2} = \frac{4 - 4yi - y^2}{4 + y^2}$$
13. **Rewrite numerator:**
$$4 - y^2 - 4yi$$
14. **Equate real and imaginary parts:**
$$\frac{4 - y^2}{4 + y^2} = \frac{3}{5}, \quad -\frac{4y}{4 + y^2} = \frac{1}{5}$$
15. **From imaginary part:**
$$-\frac{4y}{4 + y^2} = \frac{1}{5} \implies -20y = 4 + y^2 \implies y^2 + 20y + 4 = 0$$
16. **Solve quadratic:**
$$y = \frac{-20 \pm \sqrt{400 - 16}}{2} = \frac{-20 \pm \sqrt{384}}{2} = \frac{-20 \pm 8\sqrt{6}}{2} = -10 \pm 4\sqrt{6}$$
17. **Check real part with these values:**
$$\frac{4 - y^2}{4 + y^2} = \frac{3}{5}$$
18. **Calculate $y^2$ for $y = -10 + 4\sqrt{6}$:**
$$y^2 = (-10 + 4\sqrt{6})^2 = 100 - 80\sqrt{6} + 96 = 196 - 80\sqrt{6}$$
19. **Calculate left side:**
$$\frac{4 - y^2}{4 + y^2} = \frac{4 - (196 - 80\sqrt{6})}{4 + (196 - 80\sqrt{6})} = \frac{4 - 196 + 80\sqrt{6}}{4 + 196 - 80\sqrt{6}} = \frac{-192 + 80\sqrt{6}}{200 - 80\sqrt{6}}$$
20. **Approximate numerator and denominator:**
$$-192 + 80 \times 2.449 = -192 + 195.92 = 3.92$$
$$200 - 80 \times 2.449 = 200 - 195.92 = 4.08$$
21. **Ratio:**
$$\frac{3.92}{4.08} \approx 0.96$$
22. **Compare to $\frac{3}{5} = 0.6$:** Not equal, so try the other root.
23. **For $y = -10 - 4\sqrt{6}$:**
$$y^2 = (-10 - 4\sqrt{6})^2 = 196 + 80\sqrt{6}$$
24. **Calculate left side:**
$$\frac{4 - y^2}{4 + y^2} = \frac{4 - (196 + 80\sqrt{6})}{4 + (196 + 80\sqrt{6})} = \frac{-192 - 80\sqrt{6}}{200 + 80\sqrt{6}}$$
25. **Approximate numerator and denominator:**
$$-192 - 80 \times 2.449 = -192 - 195.92 = -387.92$$
$$200 + 80 \times 2.449 = 200 + 195.92 = 395.92$$
26. **Ratio:**
$$\frac{-387.92}{395.92} \approx -0.98$$
27. **Not equal to $0.6$, so no solution here.**
28. **Conclusion:** The only consistent solution is from step 3: $x=2$ and from step 10: $y = -\frac{1}{4}$.
29. **Final answer:**
$$Z = 2 - \frac{1}{4}i$$