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Câu hỏi: Cho hai số phức ${{z}_{1}}=1+2i$, ${{z}_{2}}=3-i$. Tìm số phức $z=\dfrac{{{z}_{2}}}{{{z}_{1}}}$.
A. $z=\dfrac{1}{5}+\dfrac{7}{5}i$.
B. $z=\dfrac{1}{10}+\dfrac{7}{10}i$.
C. $z=\dfrac{1}{5}-\dfrac{7}{5}i$.
D. $z=-\dfrac{1}{10}+\dfrac{7}{10}i$.
A. $z=\dfrac{1}{5}+\dfrac{7}{5}i$.
B. $z=\dfrac{1}{10}+\dfrac{7}{10}i$.
C. $z=\dfrac{1}{5}-\dfrac{7}{5}i$.
D. $z=-\dfrac{1}{10}+\dfrac{7}{10}i$.
Ta có $z=\dfrac{{{z}_{2}}}{{{z}_{1}}}$ $=\dfrac{3-i}{1+2i}$ $=\dfrac{1}{5}-\dfrac{7}{5}i$.
Đáp án C.