With sliders, you can change the length of the side of the triangle above. The combined degree of the angles is 180[math]^\circ.[/math][br] [br]The area of the triangle is calculated by the formula[br] [br][math]\large \textcolor{blue}{A = \dfrac 1 2\cdot \text{ base}\cdot \text{ perpendicular height }=\dfrac 1 2 ah.}[/math][br] [br]The perpendicular height is always against the base. In the applet below, the perpendicular height is drawn with a dotted line. It always comes at right angles to the base. Right angle means 90[math]^\circ[/math] angle.[br] [br][color=#0000ff]Perimeter[/color] is obtained by adding the lengths of all sides together.

A triangle is called a [color=#0000ff] acute-angled if all its angles are less than 90[math]^\circ,[/math][/color] where [math]^\circ[/math] denotes degree. In the applet above, you can move the point B by dragging it with the mouse.

A triangle is called a [color=#0000ff] obtuse-angled if its one angle is greater than 90[math]^\circ.[/math][/color] In the applet above, you can move the point B with the slider.

One angle of a right triangle is always 90[math]^\circ[/math] and that angle is called a right angle. The sides next to the right corner are called the margins. The sides opposite the corner are called hypotenuses. In the applet above, you can change the size of the triangle using the slider.

All sides of the equilateral triangle are equal in length. All angles of the equilateral triangle are equal, i.e. 60[math]^\circ.[/math]

The two sides of the equilateral triangle are the same length and are called the sides. The third page is called the base. In the applet above, the sides starting from point A are equal in length. The height line drawn from point A divides the base into equal parts. The base angles are equal in the isosceles triangle.