Triangles

		A triangle has three sides and three angles
		The three angles always add to 180°

Equilateral, Isosceles and Scalene

There are three special names given to triangles that tell how many sides (or angles) are equal.

There can be 3, 2 or no equal sides/angles:

	Equilateral Triangle Three equal sides Three equal angles, always 60°
	Isosceles Triangle Two equal sides Two equal angles
	Scalene Triangle No equal sides No equal angles

How to remember? Alphabetically they go 3, 2, none:

• Equilateral: "equal"-lateral (lateral means side) so they have all equal sides
• Isosceles: means "equal legs", and we have two legs, right?   Also iSOSceles has two equal "Sides" joined by an "Odd" side.
• Scalene: means "uneven" or "odd", so no equal sides.

What Type of Angle?

Triangles can also have names that tell you what type of angle is inside:

	Acute Triangle All angles are less than 90°
	Right Triangle Has a right angle (90°)
	Obtuse Triangle Has an angle more than 90°

Combining the Names

Right Isosceles Triangle Has a right angle (90°), and also two equal angles Can you guess what the equal angles are?

Play With It ...

Try dragging the points around and make different triangles:

You might also like to play with the Interactive Triangle.

Angles

The three interior angles always add to 180°

Perimeter

The perimeter is the distance around the edge of the triangle: just add up the three sides:

Area

The area is half of the base times height.

• "b" is the distance along the base
• "h" is the height (measured at right angles to the base)

Area = ½ × b × h

The formula works for all triangles.

Note: a simpler way of writing the formula is bh/2

The base can be any side, Just be sure the "height" is measured at right angles to the "base":

(Note: You can also calculate the area from the lengths of all three sides using Heron's Formula.)