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Given z = (a+jb)/(c+jd), where a, b, c, d are real quantities, show that if z is real, then (a)/(b) = (c)/(d), and if z is entirely imaginary, then ac = -bd.

Mathematics

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3 parts
a

ab=cd\frac{a}{b} = \frac{c}{d}

b

ab=−dc\frac{a}{b} = -\frac{d}{c}

12

a=32,b=−52a=\frac{3}{2}, b=-\frac{5}{2}

Step-by-step solution

8 steps

Here are the solutions for the remaining questions.

Part 11· 1 step

Given z=a+jbc+jdz = \frac{a+jb}{c+jd}, where a,b,c,da, b, c, d are real quantities.

  1. Express zz in the form X+jYX + jY by multiplying the numerator and denominator by the conjugate of the denominator.

    z=a+jbc+jd×c−jdc−jdz = \frac{a+jb}{c+jd} \times \frac{c-jd}{c-jd} z=(a+jb)(c−jd)c2−(jd)2z = \frac{(a+jb)(c-jd)}{c^2 - (jd)^2} z=ac−ajd+jbc−j2bdc2+d2z = \frac{ac - ajd + jbc - j^2bd}{c^2 + d^2}

    Since j2=−1j^2 = -1:

    z=ac−ajd+jbc+bdc2+d2z = \frac{ac - ajd + jbc + bd}{c^2 + d^2}

    Group the real and imaginary terms:

    z=(ac+bd)+j(bc−ad)c2+d2z = \frac{(ac + bd) + j(bc - ad)}{c^2 + d^2} z=ac+bdc2+d2+jbc−adc2+d2z = \frac{ac + bd}{c^2 + d^2} + j\frac{bc - ad}{c^2 + d^2}

Part a

ab=cd\frac{a}{b} = \frac{c}{d}

If zz is real, its imaginary part must be zero.

  1. Set the imaginary part of zz to zero.

    bc−adc2+d2=0\frac{bc - ad}{c^2 + d^2} = 0

    Since c2+d2≠0c^2 + d^2 \neq 0, the numerator must be zero:

    bc−ad=0bc - ad = 0 bc=adbc = ad
  2. Rearrange the equation to show the required ratio.

    Assuming b≠0b \neq 0 and d≠0d \neq 0, divide both sides by bdbd:

    bcbd=adbd\frac{bc}{bd} = \frac{ad}{bd} cd=ab\frac{c}{d} = \frac{a}{b}

    Thus, if zz is real, then ab=cd\boxed{\frac{a}{b} = \frac{c}{d}}.

Part b

ab=−dc\frac{a}{b} = -\frac{d}{c}

If zz is entirely imaginary, its real part must be zero.

  1. Set the real part of zz to zero.

    ac+bdc2+d2=0\frac{ac + bd}{c^2 + d^2} = 0

    Since c2+d2≠0c^2 + d^2 \neq 0, the numerator must be zero:

    ac+bd=0ac + bd = 0 ac=−bdac = -bd
  2. Rearrange the equation to show the required ratio.

    Assuming b≠0b \neq 0 and c≠0c \neq 0, divide both sides by bcbc:

    acbc=−bdbc\frac{ac}{bc} = -\frac{bd}{bc} ab=−dc\frac{a}{b} = -\frac{d}{c}

    Thus, if zz is entirely imaginary, then ab=−dc\boxed{\frac{a}{b} = -\frac{d}{c}}.

Part 12

a=32,b=−52a=\frac{3}{2}, b=-\frac{5}{2}

Given (a+b)+j(a−b)=(1+j)2+j(2+j)(a+b) + j(a-b) = (1+j)^2 + j(2+j), obtain the values of aa and bb.

  1. Simplify the right-hand side of the equation.

    First, expand (1+j)2(1+j)^2:

    (1+j)2=12+2(1)(j)+j2=1+2j−1=2j(1+j)^2 = 1^2 + 2(1)(j) + j^2 = 1 + 2j - 1 = 2j

    Next, expand j(2+j)j(2+j):

    j(2+j)=2j+j2=2j−1j(2+j) = 2j + j^2 = 2j - 1

    Now, add these two results:

    (1+j)2+j(2+j)=2j+(2j−1)=−1+4j(1+j)^2 + j(2+j) = 2j + (2j - 1) = -1 + 4j
  2. Equate the real and imaginary parts of the equation.

    The original equation becomes:

    (a+b)+j(a−b)=−1+4j(a+b) + j(a-b) = -1 + 4j

    Equating the real parts:

    a+b=−1(1)a+b = -1 \quad (1)

    Equating the imaginary parts:

    a−b=4(2)a-b = 4 \quad (2)
  3. Solve the system of linear equations for aa and bb.

    Add equation (1) and equation (2):

    (a+b)+(a−b)=−1+4(a+b) + (a-b) = -1 + 4 2a=32a = 3 a=32a = \frac{3}{2}

    Substitute the value of aa into equation (1):

    32+b=−1\frac{3}{2} + b = -1 b=−1−32b = -1 - \frac{3}{2} b=−22−32b = -\frac{2}{2} - \frac{3}{2} b=−52b = -\frac{5}{2}

    The values are a=32,b=−52\boxed{a=\frac{3}{2}, b=-\frac{5}{2}}.

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