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Triangles |
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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