condition of orthogonality

Let two straight lines of the $xy$-plane have the slopes $m_1$ and $m_2$.  The lines are at right angles to each other iff $m_1$ and $m_2$ are the opposite inverses of each other, i.e. iff (http://planetmath.org/Biconditional)

$$m_1{m}_2=-1.$$

Example.  The lines  $y=(1+\sqrt{2})x$  and  $y=(1-\sqrt{2})x$  are at right angles to each other.

Title	condition of orthogonality
Canonical name	ConditionOfOrthogonality
Date of creation	2013-03-22 14:48:05
Last modified on	2013-03-22 14:48:05
Owner	pahio (2872)
Last modified by	pahio (2872)
Numerical id	14
Author	pahio (2872)
Entry type	Result
Classification	msc 15A57
Classification	msc 51F20
Synonym	condition of perpendicularity
Related topic	OrthogonalCurves
Related topic	InverseNumber
Related topic	OppositeNumber
Related topic	NormalLine
Related topic	AngleBetweenTwoLines
Related topic	PerpendicularityInEuclideanPlane
Related topic	Evolute2
Related topic	ExampleOfFindingCatacaustic
Defines	opposite inverse