Track / Stage 5 / 205 of 400 #805 of 1964
Problem 805 AIME late Algebra Difficulty 5.3 Find the answer
Given z 1 , z 2 z_{1}, z_{2} z 1 , z 2 are conjugate complex numbers, if ∣ z 1 − z 2 ∣ = 4 3 , z 1 z 2 2 ∈ R \left|z_{1}-z_{2}\right|=4 \sqrt{3}, \frac{z_{1}}{z_{2}^{2}} \in \mathbf{R} ∣ z 1 − z 2 ∣ = 4 3 , z 2 2 z 1 ∈ R , then ∣ z 1 ∣ = \left|z_{1}\right|= ∣ z 1 ∣ =
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Official solution Noticing z 2 = z 1 ‾ z_{2}=\overline{z_{1}} z 2 = z 1 , we have z 1 z 2 2 = z 1 ‾ z 2 2 = z 1 ‾ z ˉ 2 2 ⇒ z 1 ⋅ z ˉ 2 2 = z 2 2 ⋅ z 1 ‾ ⇒ z 1 3 = z 2 3 \frac{z_{1}}{z_{2}^{2}}=\frac{\overline{z_{1}}}{z_{2}^{2}}=\frac{\overline{z_{1}}}{\bar{z}_{2}^{2}} \Rightarrow z_{1} \cdot \bar{z}_{2}^{2}=z_{2}^{2} \cdot \overline{z_{1}} \Rightarrow z_{1}^{3}=z_{2}^{3} z 2 2 z 1 = z 2 2 z 1 = z ˉ 2 2 z 1 ⇒ z 1 ⋅ z ˉ 2 2 = z 2 2 ⋅ z 1 ⇒ z 1 3 = z 2 3 , thus { z 1 2 + z 1 z 2 + z 2 2 = 0 , ( z 1 − z 2 ) ( z 1 ‾ − z 2 ‾ ) = 48 ⇒ { z 1 2 + z 1 z 2 + z 2 2 = 0 , − z 1 2 + 2 z 1 z 2 − z 2 2 = 48 ⇒ ∣ z 1 ∣ 2 = z 1 z 2 = 16 \left\{\begin{array}{l}z_{1}^{2}+z_{1} z_{2}+z_{2}^{2}=0, \\ \left(z_{1}-z_{2}\right)\left(\overline{z_{1}}-\overline{z_{2}}\right)=48\end{array} \Rightarrow\left\{\begin{array}{l}z_{1}^{2}+z_{1} z_{2}+z_{2}^{2}=0, \\ -z_{1}^{2}+2 z_{1} z_{2}-z_{2}^{2}=48\end{array} \Rightarrow\left|z_{1}\right|^{2}=z_{1} z_{2}=16\right.\right. { z 1 2 + z 1 z 2 + z 2 2 = 0 , ( z 1 − z 2 ) ( z 1 − z 2 ) = 48 ⇒ { z 1 2 + z 1 z 2 + z 2 2 = 0 , − z 1 2 + 2 z 1 z 2 − z 2 2 = 48 ⇒ ∣ z 1 ∣ 2 = z 1 z 2 = 16 . Therefore, ∣ z 1 ∣ = 4 \left|z_{1}\right|=4 ∣ z 1 ∣ = 4 .
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Source: NuminaMath-1.5 ,
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