Naming by Angles
Part of the Triangles section of Coddy's Geometry journey. Lesson 3 of 63.
Triangles are also sorted by their biggest angle. In an acute triangle all three angles are under 90°. A right triangle has one 90° angle. An obtuse triangle has one angle over 90°. A triangle can never have two right or two obtuse angles, because the three angles only add up to 180°.
Often a figure shows only two angles. Then find the third first: here 180 - 35 - 25 = 120, so the angle at C is 120° and the triangle is obtuse, even though the drawing makes C look sharper than that.
Challenge
Each triangle shows two of its angles. Work out each triangle's third angle, then tap the obtuse triangle and 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