Three Sides, Three Corners
Part of the Triangles section of Coddy's Geometry journey. Lesson 1 of 63.
A triangle is three points, not on one line, joined by three segments. The points are its vertices (one is a vertex), and the segments are its sides. A triangle with vertices A, B and C is called triangle ABC, written △ABC.
Each angle of the triangle has one side it does not touch: the side across from it. In △ABC the side opposite ∠A is BC, the side opposite ∠B is CA, and the side opposite ∠C is AB. Books often name a side with the small letter of the corner it faces, so side a is BC.
To find the opposite side, take the two letters that are not the vertex. Opposite ∠B, the other letters are A and C, so the side is AC.
Challenge
Here is △KLM. Tap the side opposite ∠L, then press Check.
Try it yourself
Find the right part on the figure
Test yourselfThis lesson includes a short quiz. Start the lesson to answer it and track your progress.
All lessons in Triangles
1Sides and Names
Three Sides, Three CornersSorting by SidesNaming by AnglesTicks into EquationsThe Longest SideRecap: Sorting Triangles2The Triangle Inequality
Can Three Sticks Meet?Exactly ReachingThe Range of the Third SideThe FormulaRecap: The Third Side5The Pythagorean Theorem
Squares on the SidesFinding the HypotenusePythagorean TriplesWhen It Is Not WholeRecap: The Hypotenuse3Perimeter
Around the EdgeA Missing SideIsosceles PerimeterSides as ExpressionsRecap: Around the Edge6Finding a Leg
Working BackwardsThe LadderWhich Side Is c?A Leg That Is Not WholeRecap: Finding a Leg