CK12 - Geometry

Recallthat a right angleis shownin diagramsby usinga smallsquaremarkingin the angle,as shownbelow.

So, whenyou knowthat a triangleis right,and you havethe measureof one acuteangle,you can easily
find the other.

Example 3

Whatis the measureof the missinganglein the trianglebelow?

Sincethe triangleaboveis a right triangle,the two acuteanglesmustbe complementary. Theirsum will be

We will representthe missinganglewith the variable and writean equation.

Nowwe can use inverseoperationsto isolatethe variable,and thenwe will havethe measureof the
missingangle.

The measureof the missingangleis

ExteriorAnglesin a Triangle

One of the mostimportantlessonsyou havelearnedthus far was the trianglesum theorem,statingthat the

sum of the measureof the interioranglesin any trianglewill be equalto You know, however, that
thereare two typesof anglesformedby triangles:interiorand exterior. It may be that thereis a similarthe-
oremthat identifiesthe sum of the exterioranglesin a triangle.

Recallthat the exteriorand interioranglesarounda singlevertexsum to as shownbelow.