Difference between revisions of "Angle bisector"

Revision as of 01:33, 1 July 2020

This is an AoPSWiki Word of the Week for June 6-12

For an angle $\angle ABC$ , the (internal) angle bisector of $\angle ABC$ is the line from B such that the angle between this line and $BC$ is congruent to the angle between this line and $AB$ :

A given angle $\angle ABC$ also has an external angle bisector, which bisects external angle $ABC$ :

$[asy] import markers; pair A,B,C,D,E,F; B=(0,0); C=(5,0); A=(4,2); D=(3,4); E=(10/3,0); F=rotate(-90,A)*(E); draw(A--B--C--cycle,blue); draw(C--D,blue); pen p=blue+0.15mm; draw(A--F,p); dot(A^^B^^C,red); label("$A$",A,NE); label("$B$",B,W); label("$C$",C,E); markangle(n=1,radius=20,D,A,F,green); markangle(n=1,radius=22,F,A,B,green); [/asy]$

The external angle is defined by $\angle A + \text{ external }\angle A = 180^\circ$ , and the two angle bisectors are perpendicular to each other.

Features of Angle Bisectors

The distances from a point on an angle bisector to both of its sides are equal.
The angle bisectors are the locus of points which are equidistant from the two sides of the angle.
A reflection about either angle bisector maps the two sides of the angle to each other.
In a triangle, the Angle Bisector Theorem gives the ratio in which the angle bisector cuts the opposite side.
In a triangle, the internal angle bisectors (which are cevians) all intersect at the incenter of the triangle. The internal angle bisector of one angle and the external angle bisectors of the other two angles all intersect at an excenter of the triangle.
A bisector of an angle can be constructed using a compass and straightedge.

$[asy] size(300); import markers; pair excenter(pair A, pair B, pair C){ pair X, Z; X=A+expi((angle(A-B)+angle(C-A))/2); Z=C+expi((angle(C-B)+angle(A-C))/2); return extension(X,A,Z,C); } pair X=(0,0), Y=(-10,0), Z=(-3,6); pair exX=excenter(Z,X,Y), exY=excenter(X,Y,Z), exZ=excenter(Y,Z,X); label("X",X,2*(1.5,-1));label("Y",Y,2*(-3,-1));label("Z",Z,(0.6,2)); dot(X^^Y^^Z); draw(0.3*(X-Y)+X--Y+0.3*(Y-X));draw(0.3*(Y-Z)+Y--Z+0.3*(Z-Y));draw(0.3*(X-Z)+X--Z+0.3*(Z-X)); draw(X+0.3*(X-exX)--exX,orange);draw(Y+0.3*(Y-exY)--exY,orange);draw(Z+0.3*(Z-exZ)--0.6*(exZ-Z)+Z,orange); draw(Y-0.5*(exX-Y)--Y+0.5*(exX-Y),green);draw(Z-0.5*(exX-Z)--Z+0.5*(exX-Z),green);draw(X-0.5*(exY-X)--X+0.5*(exY-X),green); pair I=extension(X,exX,Z,exZ); dot("I",I,(-1,2)); draw(circle(I,length(I-foot(I,X,Y))),blue); markangle(n=1,I,Z,X,radius=20,marker(markinterval(stickframe(n=2),true))); [/asy]$

Triangle $\triangle XYZ$ with incenter I, with angle bisectors (orange), incircle (blue), and inradii (green)

Triangle XYZ with incenter I, with angle bisectors (orange), incircle (blue), and external angle bisectors (green)

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Difference between revisions of "Angle bisector"

Revision as of 01:33, 1 July 2020

Features of Angle Bisectors

See also

@@ Line 41: / Line 41: @@
 {{asy image|<asy>
 size(300);
+import markers;
 pair excenter(pair A, pair B, pair C){
 pair X, Z;
@@ Line 53: / Line 54: @@
 draw(0.3*(X-Y)+X--Y+0.3*(Y-X));draw(0.3*(Y-Z)+Y--Z+0.3*(Z-Y));draw(0.3*(X-Z)+X--Z+0.3*(Z-X));
 draw(X+0.3*(X-exX)--exX,orange);draw(Y+0.3*(Y-exY)--exY,orange);draw(Z+0.3*(Z-exZ)--0.6*(exZ-Z)+Z,orange);
+draw(Y-0.5*(exX-Y)--Y+0.5*(exX-Y),green);draw(Z-0.5*(exX-Z)--Z+0.5*(exX-Z),green);draw(X-0.5*(exY-X)--X+0.5*(exY-X),green);
 pair I=extension(X,exX,Z,exZ);
 dot("I",I,(-1,2));
 draw(circle(I,length(I-foot(I,X,Y))),blue);
-</asy>|center|Triangle <math>\triangle XYZ</math> with [[incenter]] ''I'', with [[angle bisector]]s (red), [[incircle]] (blue), and [[inradius|inradii]] (green)}}
+markangle(n=1,I,Z,X,radius=20,marker(markinterval(stickframe(n=2),true)));
+</asy>|center|Triangle <math>\triangle XYZ</math> with [[incenter]] ''I'', with [[angle bisector]]s (orange), [[incircle]] (blue), and [[inradius|inradii]] (green)}}
 [[Image:Incenter.PNG|left|thumb|300px|Triangle ''XYZ'' with [[incenter]] ''I'', with [[angle bisector]]s (orange), [[incircle]] (blue), and [[external angle bisector]]s (green)]]