Re: HSC 2016 3U Marathon
$\textbf{NEW QUESTION}$
$\noindent Let $\ell_1$ and $\ell_2$ be two lines in the Cartesian plane and suppose that neither line is horizontal. Let the slope of $\ell_1$ and $\ell_2$ be $m_1$ and $m_2$, respectively. Prove that $m_1 m_2 = -1$ if and only if $\ell _1$...