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Complex Numbers question

2011 · Shift 1 · Q24
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  5. /2011 · Shift 1 · Q24

Complex Numbers question

2011 · Shift 1 · Q24

JEE AdvancedMathematicsComplex NumbersNumerical+4 / −1
If z is any complex number satisfying  ∣z−3−2i∣≤2\,\left| {z - 3 - 2i} \right| \le 2∣z−3−2i∣≤2, then the minimum value of ∣2z−6+5i∣\left| {2z - 6 + 5i} \right|∣2z−6+5i∣ is
Numerical answer
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Correct answer: 5

  1. Let us rewrite the given condition in a more useful form.

Given ∣z−3−2i∣≤2|z-3-2i|\le 2∣z−3−2i∣≤2 which means ∣z−(3+2i)∣≤2.|z-(3+2i)|\le 2.∣z−(3+2i)∣≤2.

So, in the Argand plane, zzz lies inside or on the circle with:

  • center C=3+2iC=3+2iC=3+2i
  • radius r=2r=2r=2
  1. Now simplify the expression whose minimum is required:

∣2z−6+5i∣=∣2(z−3+52i)∣|2z-6+5i| = |2(z-3+\tfrac{5}{2}i)|∣2z−6+5i∣=∣2(z−3+25​i)∣ But it is cleaner to factor 222 directly:

2z−6+5i=2(z−3+52i)=2(z−(3−52i)).2z-6+5i = 2\left(z-3+\frac{5}{2}i\right)=2\left(z-(3-\tfrac{5}{2}i)\right).2z−6+5i=2(z−3+25​i)=2(z−(3−25​i)).

Hence ∣2z−6+5i∣=2∣z−(3−52i)∣.|2z-6+5i|=2\left|z-\left(3-\frac{5}{2}i\right)\right|.∣2z−6+5i∣=2​z−(3−25​i)​.

So we need the minimum of the distance from zzz to the fixed point P=3−52i,P=3-\frac{5}{2}i,P=3−25​i, multiplied by 222.

  1. Find the distance between the center C=3+2iC=3+2iC=3+2i of the allowed circle and the point P=3−52iP=3-\frac{5}{2}iP=3−25​i.

CP=∣(3+2i)−(3−52i)∣CP=\left|\left(3+2i\right)-\left(3-\frac{5}{2}i\right)\right|CP=​(3+2i)−(3−25​i)​ =∣2i+52i∣=∣92i∣=92.=\left|2i+\frac{5}{2}i\right|=\left|\frac{9}{2}i\right|=\frac{9}{2}.=​2i+25​i​=​29​i​=29​.

  1. The minimum distance from a fixed point outside a circle to any point on or inside the circle is:

CP−rCP-rCP−r provided CP>rCP>rCP>r.

Here, CP=92,r=2,CP=\frac{9}{2}, \qquad r=2,CP=29​,r=2, so min⁡∣z−(3−52i)∣=92−2=52.\min \left|z-\left(3-\frac{5}{2}i\right)\right|=\frac{9}{2}-2=\frac{5}{2}.min​z−(3−25​i)​=29​−2=25​.

  1. Therefore,

min⁡∣2z−6+5i∣=2⋅52=5.\min |2z-6+5i|=2\cdot \frac{5}{2}=5.min∣2z−6+5i∣=2⋅25​=5.

  1. Final answer:

5\boxed{5}5​

Comparison with stored correct answer:

  • Derived answer = 555
  • Stored correct answer = 555
  • They agree.
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