JEE Main & Advanced Mathematics Complex Numbers and Quadratic Equations Question Bank Geometry of complex numbers

  • question_answer
    If \[z=x+iy,\] then area of the triangle whose vertices are points \[z,\,iz\] and \[z+iz\] is [MP PET 1997; IIT 1986; AMU 2000; UPSEAT 2002]

    A) \[2|z{{|}^{2}}\]

    B) \[\frac{1}{2}|z{{|}^{2}}\]

    C) \[|z{{|}^{2}}\]

    D) \[\frac{3}{2}|z{{|}^{2}}\]

    Correct Answer: B

    Solution :

    Let\[z=x+iy\]; \[z+iz=(x-y)+i(x+y)\]and\[iz=-y+ix\] If A denotes the area of the triangle formed by \[z,z+iz\] and \[iz\], then \[A=\frac{1}{2}\left| \begin{matrix}    x & y & 1  \\    x-y & x+y & 1  \\    -y & x & 1  \\ \end{matrix} \right|\] Applying transformation\[{{R}_{2}}\to {{R}_{2}}-{{R}_{1}}-{{R}_{3}}\], we get \[A=\frac{1}{2}\left| \begin{matrix}    x & y & 1  \\    0 & 0 & -1  \\    -y & x & 0  \\ \end{matrix} \right|=\frac{1}{2}({{x}^{2}}+{{y}^{2}})=\frac{1}{2}|z{{|}^{2}}\]


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