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If Z = a + I B Lies in Third Quadrant, Then ¯ Z Z Also Lies in Third Quadrant If

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प्रश्न

If \[z = a + ib\]  lies in third quadrant, then \[\frac{\bar{z}}{z}\] also lies in third quadrant if

विकल्प

  • \[a > b > 0\]

  • \[a < b < 0\]

  • \[b < a < 0\]

  • \[b > a > 0\]

MCQ
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उत्तर

Since, \[z = a + ib\] lies in third quadrant. \[\Rightarrow a < 0 \text { and } b < 0 . . . . (1)\]

Now,

\[\frac{\bar{z}}{z} = \frac{\bar{{a + ib}}}{a + ib}\]

\[ = \frac{a - ib}{a + ib}\]

\[ = \frac{a - ib}{a + ib} \times \frac{a - ib}{a - ib}\]

\[ = \frac{a^2 + i^2 b^2 - 2abi}{a^2 - i^2 b^2}\]

\[ = \frac{a^2 - b^2 - 2abi}{a^2 + b^2}\]

Since, 

\[\frac{\bar{z}}{z}\] also lies in third quadrant.

\[\Rightarrow a^2 - b^2 < 0\]

\[ \Rightarrow (a - b)(a + b) < 0\]

\[ \Rightarrow a - b > 0 \text { and  }a + b < 0\]

\[ \Rightarrow a > b . . . . (2)\]

From (1) and (2),

\[b < a < 0\]

Hence, the correct option is (c).

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अध्याय 13: Complex Numbers - Exercise 13.6 [पृष्ठ ६६]

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आर.डी. शर्मा Mathematics [English] Class 11
अध्याय 13 Complex Numbers
Exercise 13.6 | Q 37 | पृष्ठ ६६

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