Type of AngleDescription
Acute Anglean angle that is less than 90°
Right Anglean angle that is 90° exactly
Obtuse Anglean angle that is greater than 90° but
less than 180°
Straight Anglean angle that is 180° exactly
Reflex Anglean angle that is greater than 180°
Try It Yourself:

© 2015 v0.86

In One Diagram

This diagram might make it easier to remember:
Also: Acute, Obtuse and Reflex are in alphabetical order.  

Also: the letter "A" has an acute angle.

Be Careful What You Measure

Obtuse AngleReflex Angle
This is an Obtuse Angle
And this is a Reflex Angle
But the lines are the same ... so when naming the angles make sure
that you know which angle is being asked for!

Positive and Negative Angles

When measuring from a line:
  • positive angle goes counterclockwise (opposite direction that clocks go)
  • negative angle goes clockwise

Example: −67°

Play With It ...

Try dragging the points around and make different triangles:

You might also like to play with the Interactive Triangle.


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


triangle b h
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

Example: What is the area of this triangle?

(Note: 12 is the height, not the length of the left-hand side)

Height = h = 12
Base = b = 20
Area = ½ × b × h = ½ × 20 × 12 = 120
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.)

Why is the Area "Half of bh"?

Imagine you "doubled" the triangle (flip it around one of the upper edges) to make a square-like shape (a parallelogram) which can be changed to a simplerectangle:
triangle area
THEN the whole area is bh, which is for both triangles, so just one is ½ × bh

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