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Complex Numbers question

2024 · 9 Apr · Shift 1 · Q57
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Complex Numbers question

2024 · 9 Apr · Shift 1 · Q57

JEE MainMathematicsComplex NumbersNumerical+4 / −1
The sum of the square of the modulus of the elements in the set {z=a+ib:a,b∈Z,z∈C,∣z−1∣≤1,∣z−5∣≤∣z−5i∣}\{z=\mathrm{a}+\mathrm{ib}: \mathrm{a}, \mathrm{b} \in \mathbf{Z}, z \in \mathbf{C},|z-1| \leq 1,|z-5| \leq|z-5 \mathrm{i}|\}{z=a+ib:a,b∈Z,z∈C,∣z−1∣≤1,∣z−5∣≤∣z−5i∣} is ‾\underline{\hspace{2cm}}​.
Numerical answer
View written solutionFree

Correct answer: 9

  1. We need all complex numbers of the form

    \quad a,b\in \mathbb Z$$ satisfying both $$|z-1|\le 1$$ and $$|z-5|\le |z-5i|.$$
  2. First condition: ∣z−1∣≤1|z-1|\le 1∣z−1∣≤1 means ∣(a−1)+ib∣≤1|(a-1)+ib|\le 1∣(a−1)+ib∣≤1 so (a−1)2+b2≤1. (a-1)^2+b^2\le 1.(a−1)2+b2≤1.

    Since a,ba,ba,b are integers, we find all integer solutions of (a−1)2+b2≤1. (a-1)^2+b^2\le 1.(a−1)2+b2≤1.

    Possible values:

    • (a−1)2=0⇒a=1(a-1)^2=0 \Rightarrow a=1(a−1)2=0⇒a=1, then b2≤1⇒b=0,±1b^2\le 1 \Rightarrow b=0,\pm 1b2≤1⇒b=0,±1
    • (a−1)2=1⇒a=0(a-1)^2=1 \Rightarrow a=0(a−1)2=1⇒a=0 or 222, then b2≤0⇒b=0b^2\le 0 \Rightarrow b=0b2≤0⇒b=0

    Hence possible lattice points are: z∈{1,  1+i,  1−i,  0,  2}.z\in \{1,\;1+i,\;1-i,\;0,\;2\}.z∈{1,1+i,1−i,0,2}.

  3. Second condition: ∣z−5∣≤∣z−5i∣.|z-5|\le |z-5i|.∣z−5∣≤∣z−5i∣. Put z=a+ibz=a+ibz=a+ib. Then ∣z−5∣2=(a−5)2+b2,|z-5|^2=(a-5)^2+b^2,∣z−5∣2=(a−5)2+b2, ∣z−5i∣2=a2+(b−5)2.|z-5i|^2=a^2+(b-5)^2.∣z−5i∣2=a2+(b−5)2.

    So the inequality becomes (a−5)2+b2≤a2+(b−5)2. (a-5)^2+b^2\le a^2+(b-5)^2.(a−5)2+b2≤a2+(b−5)2.

    Expanding: a2−10a+25+b2≤a2+b2−10b+25a^2-10a+25+b^2\le a^2+b^2-10b+25a2−10a+25+b2≤a2+b2−10b+25 −10a≤−10b-10a\le -10b−10a≤−10b a≥b.a\ge b.a≥b.

  4. Now test the candidate points from step 2:

    • z=1z=1z=1 gives (a,b)=(1,0)(a,b)=(1,0)(a,b)=(1,0), and 1≥01\ge 01≥0 ✓
    • z=1+iz=1+iz=1+i gives (1,1)(1,1)(1,1), and 1≥11\ge 11≥1 ✓
    • z=1−iz=1-iz=1−i gives (1,−1)(1,-1)(1,−1), and 1≥−11\ge -11≥−1 ✓
    • z=0z=0z=0 gives (0,0)(0,0)(0,0), and 0≥00\ge 00≥0 ✓
    • z=2z=2z=2 gives (2,0)(2,0)(2,0), and 2≥02\ge 02≥0 ✓

    So all five points satisfy both conditions.

  5. Now compute the sum of the squares of their moduli.

    Recall ∣a+ib∣2=a2+b2.|a+ib|^2=a^2+b^2.∣a+ib∣2=a2+b2.

    Therefore:

    • For z=1z=1z=1: ∣z∣2=1|z|^2=1∣z∣2=1
    • For z=1+iz=1+iz=1+i: ∣z∣2=12+12=2|z|^2=1^2+1^2=2∣z∣2=12+12=2
    • For z=1−iz=1-iz=1−i: ∣z∣2=12+(−1)2=2|z|^2=1^2+(-1)^2=2∣z∣2=12+(−1)2=2
    • For z=0z=0z=0: ∣z∣2=0|z|^2=0∣z∣2=0
    • For z=2z=2z=2: ∣z∣2=4|z|^2=4∣z∣2=4

    Sum: 1+2+2+0+4=9.1+2+2+0+4=9.1+2+2+0+4=9.

  6. Hence the required integer is 9.\boxed{9}.9​.

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