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A triangle is a three-sided figure. The sum of the interior angles of a triangle is 180 degrees. Each of the three points on the tips of a triangle is called a vertex (the plural of vertex is vertices).

The perimeter of a triangle is the sum of the lengths of the three sides. The area of a triangle is (1/2)*base*height.

Triangles can be classified by the length of their side or by their interior angles.

Classifying triangles by side length:

equilateral triangle The sides of an equilateral triangle are all the same length and all the interior angles are 60 degrees.

isosceles triangle An isosceles triangle has two sides that are the same length and two interior angles that are the same.

scalene triangle The sides of a scalene triangle are all different lengths. A right triangle can be a scalene triangle (but not all right triangles are scalene triangles).

The interior angles of a all triangles add up to 180 degrees. Every triangle has at least two acute angles (an acute angle is less than 90 degrees).

Classifying triangles by interior angles:

acute triangle All three interior angles of an acute triangle are acute (less than 90 degrees). Equilateral triangles are acute triangles.

obtuse triangle One of the interior angles of an obtuse triangle is obtuse ( an obtuse angle is over 90 degrees).

right triangle A right triangle has one angle that is a right angle (a right angle is a 90-degree angle). The Pythagorean theorem states that for a right triangle, the sum of the squares of the two shorter sides is equal to the square of the hypotenuse (h): a2 + b2 = h2. The hypotenuse is the longest side of a right triangle, and is the side opposite the right angle.

Triangle Worksheets:

Triangle Worksheet Read about triangles, do dot-to-dots, draw them, color them, and write the names of things with a triangular shape. Or go to the answers.

Triangle Classification Worksheet Read about how to classify triangles with respect to side length and interior angles (equilateral, isosceles, scalene, right, acute, and obtuse triangles). Or go to the answers.

Angles and Triangles Label the angles (acute, right, obtuse, and straight) and triangles (equilateral, isosceles, scalene, and right). Answers

Finish the Drawing Worksheets

Symmetrical Triangle Picture #1: Finish the Drawing Worksheet Finish the drawing of the triangle around the line of symmetry. Or go to the answer page.	Symmetrical Triangle Picture #2: Finish the Drawing Worksheet Finish the drawing of the triangle around the line of symmetry. Or go to the answer page.	Symmetrical Triangle Picture #1: Finish the Drawing and Fill in the Missing Letters Worksheet Finish the drawing of the triangle around the line of symmetry. Or go to the answer page.	Symmetrical Triangle Picture #2: Finish the Drawing and Fill in the Missing Letters Worksheet Finish the drawing of the triangle around the line of symmetry. Or go to the answer page.

Books to Print

Things Shaped Like Triangles A Printable Activity Book A small, triangular shaped book with triangular things to draw, including mountain, volcano, rocket, banner, and pyramid. A one-page printout.

Triangles A Printable Activity Book A small, triangular shaped book about triangles, with pages: circle the triangles, draw a triangle, definition of an equilateral triangle, definition of an isosceles triangle, and definition of a right triangle. A one-page printout.

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