NotationNotes:1) Anglescan be namedby a number, a singleletterat the vertex,or by the threepoints
that form the angle.Whenan angleis namedwith threeletters,the middleletterwill alwaysbe the vertex
of the angle.In the diagramabove,the anglecan be written , or , or. You can use
one letterto namethis anglesincepoint is the vertexand thereis only one angleat point. 2) If two
or moreanglessharethe samevertex,you MUSTuse threelettersto namethe angle.For example,in the
imagebelowit is unclearwhichangleis referredto by. To talk aboutthe anglewith one arc, you would
write. For the anglewith two arcs,you’dwrite.We use a ruler to measuresegmentsby theirlength.But how do we measurean angle?The lengthof the
sidesdoesnot changehow wide an angleis “open.”Insteadof usinglength,the size of an angleis measured
by the amountofrotationfrom one side to another. By definition,a full turn is definedas 360 degrees.Use
the symbol° for degrees.You may haveheard“360”usedas slangfor a “full circle”turn, and this expression
comesfrom the fact that a full rotationis 360°.
The angle that is madeby rotating through one-fourthof a full turn is very special. It measures
and we call this arightangle.Rightanglesare easy to identify, as they look like the corners
of mostbuildings,or a cornerof a pieceof paper.
Arightanglemeasuresexactly90º.
Rightanglesare usuallymarkedwith a smallsquare.Whentwo lines,two segments,or two rays intersect
at a right angle,we say that they areperpendicular. The symbol is usedfor two perpendicularlines.