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angle between two lines (Definition)

The angle between two lines in a plane is defined to be

If $ \theta$ denotes the angle between two lines, it always satisfies the inequalities
$\displaystyle 0 \leqq \theta \leqq \frac{\pi}{2}.$ (1)

If the slopes of the two lines are $ m_1$ and $ m_2$, the angle $ \theta$ is obtained from
$\displaystyle \tan\theta = \left\vert\frac{m_1-m_2}{1+m_1m_2}\right\vert.$ (2)

This equation clicks in the case that $ m_1m_2 = -1$, when the lines are perpendicular and $ \theta$ equals to $ \displaystyle\frac{\pi}{2}$. Also, if one of the lines is parallel to $ y$-axis, it has no slope; then the angle $ \theta$ must be deduced using the slope of the other line.

If one of the slopes is 0, the angle between the two lines is just the angle between one of the lines and the $ x$-axis. Assume the other line has slope $ m$, then formula (2) above becomes

$\displaystyle \tan\theta = \left\vert m\right\vert.$ (3)

If, on the other hand, one of the slopes is infinite, meaning that the line is parallel to the $ y$-axis, then the angle between two lines is the same as the angle between one line (with slope $ m$) and the $ y$-axis, which is

$\displaystyle \tan\theta = \left\vert\frac{1}{m}\right\vert.$ (4)

The above formula is consistent with formula (2) in the sense that if we let one of $ m_1$ or $ m_2$ approach $ \infty$, we get formula (4).

Remark. If both slopes are positive, then formula (2) above is really just a disguised form of the subtraction formula for tangent. In diagram, this means


\begin{pspicture}(-5,-1)(7,4) \psaxes[Dx=20,Dy=20]{->}(-1,0)(-3,-1)(6,4) \psline... ...8){$\l_2$} \rput(5.2,2.8){$\l_1$} \rput(-3,0){.} \rput(-1,-1){.} \end{pspicture}

In the diagram above, we see that the angle between the two lines is the algebraic difference of the two angles made between each of the lines and the $ x$-axis.

In the Euclidean space, the angle $ \theta$ between two lines is most comfortably defined by using the direction vectors $ \vec{u}$ and $ \vec{v}$ of the lines:

$\displaystyle \cos\theta = \left\vert\frac{\vec{u}\cdot\vec{v}}{\vert\vec{u}\vert\vert\vec{v}\vert}\right\vert$
Also this angle satisfies (1). The angle given by the cosine can be interpreted to be formed after translating the one line in the space, without to alter its direction, to such a location that it intersects the other line -- then both lines are in the same plane, and one may think that the angle is defined as in the beginning of this entry.

Remark. The angle between two curves which intersect each other in a point $ P$ means the angle between the tangent lines of the curves in $ P$; such angle may always be chosen acute or right. For example, the exponential curves $ y = a^x$ and $ y = b^x$ intersect each other under the angle $ \theta$ with $ \tan\theta = \vert\frac{\ln{a}-\ln{b}}{1+\ln{a}\ln{b}}\vert$, if $ a$ is not the inverse number of $ b$.



"angle between two lines" is owned by pahio. [ full author list (2) ]
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See Also: addition and subtraction formulas for tangent, condition of orthogonality, conformal mapping, angle between two planes, distance of non-parallel lines, mutual positions of vectors, line in space, perpendicularity in Euclidean plane, angle between line and plane, inner product space

Also defines:  angle between two curves

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angle between line and plane (Definition) by pahio
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Cross-references: inverse number, exponential, right, acute, curves, tangent lines, cosine, direction vectors, Euclidean space, difference, algebraic, subtraction formula for tangent, positive, consistent, infinite, perpendicular, equation, slopes, inequalities, lying on, point, intersection, sides, angle, parallel, lines, plane
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This is version 18 of angle between two lines, born on 2007-05-29, modified 2007-10-25.
Object id is 9485, canonical name is AngleBetweenTwoLines.
Accessed 8054 times total.

Classification:
AMS MSC51N20 (Geometry :: Analytic and descriptive geometry :: Euclidean analytic geometry)

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