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α-iβ=1/(a-ib)

then prove that(α22)(a2+b2)=1

 Mar 24, 2015

Best Answer 

 #2
avatar+892 
+13

On the assumption that alpha, beta, a and b are real,

$$\alpha -\imath \beta=\frac{1}{a-\imath b}=\frac{a+\imath b}{a^{2}+b^{2}}.$$

Taking the complex conjugate of both sides,

$$\alpha + \imath\beta=\frac{a-\imath b}{a^{2}+b^{2}},$$

and multiplying the equations together,

$$\alpha^{2}+\beta^{2}=\frac{a^{2}+b^{2}}{(a^{2}+b^{2})^{2}},$$

from which the result follows.

 Mar 24, 2015
 #1
avatar+2353 
+10

I think you either forgot to mention some of the details, or the variables (other than i off course) have an implicit meaning which I'm unaware of. 

Anyway, I can make it a little easier for you by doing the following. 

$$\begin{array}{lcl}
\alpha - i\beta &= \frac{1}{a-ib}\\
\alpha - i\beta &= \frac{1}{a-ib}\frac{a+ib}{a+ib}\\
\alpha - i\beta &= \frac{a+ib}{a^2+ aib - aib -i^2b^2}\\
\alpha - i\beta &= \frac{a+ib}{a^2+b^2}\\
(\alpha - i\beta)(\alpha + i\beta) &= \frac{(a+ib)(\alpha + i\beta)}{a^2+b^2}\\
\alpha^2 +i\alpha \beta - i\alpha \beta - i^2 \beta^2 &= \frac{(a+ib)(\alpha + i\beta)}{a^2+b^2}\\
\alpha^2 + \beta^2 &= \frac{(a+ib)(\alpha + i\beta)}{a^2+b^2}\\
(\alpha^2 + \beta^2)(a^2+b^2)&= (a+ib)(\alpha + i\beta)
\end{array}$$

 Now if you can prove that

$$(a+ib)(\alpha + i\beta) = 1$$

You're there.

 

Reinout

 Mar 24, 2015
 #2
avatar+892 
+13
Best Answer

On the assumption that alpha, beta, a and b are real,

$$\alpha -\imath \beta=\frac{1}{a-\imath b}=\frac{a+\imath b}{a^{2}+b^{2}}.$$

Taking the complex conjugate of both sides,

$$\alpha + \imath\beta=\frac{a-\imath b}{a^{2}+b^{2}},$$

and multiplying the equations together,

$$\alpha^{2}+\beta^{2}=\frac{a^{2}+b^{2}}{(a^{2}+b^{2})^{2}},$$

from which the result follows.

Bertie Mar 24, 2015

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