# Toronto Math Forum

## MAT334-2018F => MAT334--Tests => Quiz-1 => Topic started by: Victor Ivrii on September 28, 2018, 04:19:22 PM

Title: Q1: TUT 5301
Post by: Victor Ivrii on September 28, 2018, 04:19:22 PM
Show that the two lines $\Re(az + b) = 0$ and $\Re(cz + d) = 0$ are perpendicular if and only if $\Re(a\bar{c}) = 0$.
Title: Re: Q1: TUT 5301
Post by: Vedant Shah on September 28, 2018, 06:00:36 PM
Let the lines $Re(a+ib)=0$ and $Re(c+id)=0$ be perpendicular.
From section 1.2: Let $a = A+iB$ and $c= C+iD$. Then the lines are $Ax-By+Re(b)=0$ and $Cx-Dy+Re(d)=0$
Setting the slope of the first equal to the negative reciprocal of the other we get: $\frac{A}{B} = - \frac{D}{C} \iff AC=-BD$
Finally, $Re(a \bar{c}) = Re[(A+iB)(C-iD)]=AC+BD=-BD+BD=0$