The vertices of a triangle are conventionally labeled $A, B, C$ (or with other uppercase letters), and the sides with lowercase letters corresponding to the opposite vertex, as above. That is, the length of the altitude so defined. The height of a triangle is the length of a perpendicular from the apex to whichever side has been chosen as its base. In the above diagram, if $AC$ is taken to be the base of $\triangle ABC$, then $B$ is the apex. Having selected one side of a triangle to be the base, the opposite vertex to that base is called the apex. In the above diagram, it would be conventional for the side $AC$ to be identified as the base. The usual practice is that the triangle is drawn so that the base is made horizontal, and at the bottom. Thus, each vertex has an opposite side, and each side has an opposite vertex.įor a given triangle, one of the sides can be distinguished as being the base. The side of a triangle which is not one of the sides adjacent to a particular vertex is referred to as its opposite. Similarly, the two vertices of a triangle to which a particular side contributes are referred to as adjacent to that side. The two sides of a triangle which form a particular vertex are referred to as adjacent to that angle. A triangle is a polygon with exactly three sides.īecause it is a polygon, it follows that it also has three vertices and three angles.
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