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Solve the quadratic equation \[ z^{2}-6 z+10+(z-6) i=0 \] If \( z_{1} \) and \( z_{2} \) are the solutions, then \[ \left|z_{1}\right|^{2}+\left|z_{2}\right|^{2}= \]

Ask by Moran Hodges. in the United Arab Emirates
Nov 09,2024

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\[ \left|z_{1}\right|^{2}+\left|z_{2}\right|^{2} = 18 + 2\sqrt{37}. \]

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To solve the quadratic equation \[ z^{2} - 6z + 10 + (z - 6)i = 0, \] we can rewrite this as a standard quadratic equation in terms of \( z \): \[ z^2 - (6 - i)z + 10 - 6i = 0. \] Here, we identify \( a = 1 \), \( b = -(6 - i) \), and \( c = 10 - 6i \). Now, we can apply the quadratic formula: \[ z = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a}. \] Calculating \( b^2 \): \[ b^2 = (6 - i)^2 = 36 - 12i + (-1) = 35 - 12i. \] Now we calculate \( 4ac \): \[ 4ac = 4 \cdot 1 \cdot (10 - 6i) = 40 - 24i. \] Next, we compute \( b^2 - 4ac \): \[ b^2 - 4ac = (35 - 12i) - (40 - 24i) = -5 + 12i. \] Now we find \( \sqrt{b^2 - 4ac} = \sqrt{-5 + 12i} \). To compute this, we express it in polar form. Let's denote \( r = | -5 + 12i | \): \[ r = \sqrt{(-5)^2 + (12)^2} = \sqrt{25 + 144} = \sqrt{169} = 13. \] The argument \( \theta \) is given by \[ \theta = \tan^{-1}\left(\frac{12}{-5}\right) = \tan^{-1}\left(-\frac{12}{5}\right) + \pi \quad \text{(since we are in the second quadrant)}. \] Now, we need the angle corresponding to this, but it's sufficient for us to find the square root: \[ \sqrt{-5 + 12i} = \sqrt{13} \left( \cos\left(\frac{\theta}{2}\right) + i \sin\left(\frac{\theta}{2}\right) \right). \] Next, substituting back into the quadratic formula: \[ z = \frac{(6 - i) \pm \sqrt{-5 + 12i}}{2}. \] However, for our original question, we only need to calculate \[ |z_1|^2 + |z_2|^2. \] A property of the roots of a quadratic \( z^2 + bz + c = 0 \) is that \[ |z_1|^2 + |z_2|^2 = |b|^2 - 2c. \] Thus, we compute: \[ |b|^2 = |-(6 - i)|^2 = |6 - i|^2 = 36 + 1 = 37, \] and \[ -2c = -2(10 - 6i) = -20 + 12i. \] Finally, we add these to find: \[ |z_1|^2 + |z_2|^2 = 37 + |10 - 6i|^2 = 37 + (10^2 + (-6)^2) = 37 + 100 + 36 = 173. \] Thus, the final value is: \[ \boxed{173}. \]

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