Imaginary to Imaginary and Real to Real Divisions? No Way!

Algebra Level 3

Let a + b i a+bi and c + d i c+di denote two nonzero, complex numbers such that

  • a , b , c , d a,b,c,d are real numbers
  • c , d 0. c,d \neq 0.

True or False?

There exists at least one solution ( a , b , c , d ) (a,b,c,d) that satisfies the following equation: a + b i c + d i = a c + b d i . \dfrac{a + bi}{c + di} = \dfrac{a}{c} + \dfrac{b}{d}i.

Note: i = 1 i = \sqrt{-1} is the imaginary unit .

False True

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2 solutions

Assume there exists at least a solution of ( a , b , c , d ) (a,b,c,d) that satisfies the following equation

a + b i c + d i = a c + b d i a + b i = ( c + d i ) × ( a c + b d i ) = ( a b ) + ( b c d + a d c ) i Equating real parts we get, a = ( a b ) b = 0 ( 1 ) Equating imaginary parts we get, b = ( b c d + a d c ) 0 = a d c from ( 1 ) since d 0 we must have a = 0 ( a + b i ) = 0 + 0 i Since a + b i is a nonzero complex number, our assumption that a solution exists is wrong. Thus there exists no solution for the given equation. \begin{aligned}\dfrac{a + bi}{c + di}&= \dfrac{a}{c} + \dfrac{b}{d}i\\\\ \implies a+bi&=(c+di)\times \left(\dfrac{a}{c} +\dfrac{b}{d}i\right)\\ &=(a-b)+\left(\dfrac{bc}{d}+\dfrac{ad}{c}\right)i\\ \text{Equating} &\text{ real parts we get,}\\ a&=(a-b)\\ \implies b&=0\hspace{5mm}\color{#3D99F6}\small(1)\\ \text{Equating} &\text{ imaginary parts we get,}\\ b&=\left(\dfrac{bc}{d}+\dfrac{ad}{c}\right)\\ 0&=\dfrac{ad}{c}\hspace{5mm}\color{#3D99F6}\small\text{from}(1)\\ \text{since }d\neq0 &\text{ we must have }a=0\\ \implies (a+bi)&=0+0i\\ \text{Since } a+bi &\text{ is a nonzero complex number, our assumption that a solution exists is wrong.}\\ \text{Thus there} &\text{ exists no solution for the given equation.}\end{aligned}

Tom Engelsman
Oct 4, 2017

Let's rationalize the first quotient of complex numbers according to:

a + b i c + d i c d i c d i = ( a + b i ) ( c d i ) c 2 + d 2 = a c + b d c 2 + d 2 + b c a d c 2 + d 2 i \frac{a+bi}{c+di} \cdot \frac{c-di}{c-di} = \frac{(a+bi)(c-di)}{c^2 + d^2} = \frac{ac + bd}{c^2 + d^2} + \frac{bc-ad}{c^2+d^2} i (i).

We now require matching the real and the imaginary parts above:

a c + b d c 2 + d 2 = a c \frac{ac + bd}{c^2 + d^2} = \frac{a}{c} & b c a d c 2 + d 2 = b d \frac{bc-ad}{c^2+d^2} = \frac{b}{d} ;

or b c = a d bc = ad (REAL) & b c d a d 2 = b c 2 + b d 2 bcd - ad^{2} = bc^2 + bd^2 (IMAGINARY) (ii). Substituting the real expression into the imaginary side simplifies the latter into:

b c d a d 2 = b c 2 + b d 2 ( a d ) d a d 2 = ( a d ) c + b d 2 a d 2 a d 2 = d ( a c + b d ) 0 = d ( a c + b d ) bcd - ad^{2} = bc^2 + bd^2 \Rightarrow (ad)d - ad^2 = (ad)c + bd^2 \Rightarrow ad^2 - ad^2 = d(ac + bd) \Rightarrow 0 = d(ac + bd) (iii)

Since d 0 d \ne 0 , we require a c + b d = 0 d = a c b ac + bd = 0 \Rightarrow d = -\frac{ac}{b} (iv). A final substitution back into the real expression of (ii) now yields:

b c = a d b c = a ( a c b ) b 2 c = a 2 c a 2 + b 2 = 0 a = b = 0 bc = ad \Rightarrow bc = a (-\frac{ac}{b}) \Rightarrow b^{2}c = -a^{2}c \Rightarrow a^2 + b^2 = 0 \Rightarrow a = b = 0

which makes the original complex number a + b i = 0 a + bi = 0 (CONTRADICTION).

That can easily be done with z = r e θ i z = re^{\theta i} , but your way is fine and good! :)

Michael Huang - 3 years, 8 months ago

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