The trianglein the diagramhas two acuteangles.But, so is an obtuseangle.
If the anglemeasurewerenot givenyou couldcheckthis usingthe cornerof a pieceof notebookpaperor
by measuringthe anglewith a protractor. An obtuseanglewill be greaterthan 90º (the squarecornerof a
paper)and less than 180º (a straightline).Sinceone anglein the triangleaboveis obtuse,it is an obtuse
triangle.
Classifyingby SideLengths
Thereare moretypesof triangleclassesthat are not basedon anglemeasure.Instead,theseclassifications
haveto do with the sidesof the triangleand their relationshipsto eachother. Whena trianglehas all sides
of differentlength,it is calledascalenetriangle.
Whenat leasttwo sidesof a triangleare congruent,the triangleis said to be anisoscelestriangle.
Finally, whena trianglehas sidesthat are all congruent,it is calledanequilateraltriangle. Notethat by the
definitions,an equilateraltriangleis also an isoscelestriangle.
Example 4
Whichterm best describesthe trianglebelow?