Equilateral triangle

§ The area is A=a^2\frac{\sqrt{3}}{4}

§

§ The perimeter is p=3a\,\!

§ The radius of the <http://en.wikipedia.org/wiki/Circumscribed_circle>
circumscribed circle is R=a\frac{\sqrt{3}}{3}

§ The radius of the <http://en.wikipedia.org/wiki/Inscribed_circle>
inscribed circle is r=a\frac{\sqrt{3}}{6}

§ And the <http://en.wikipedia.org/wiki/Altitude_(triangle)> altitude is
h=a\frac{\sqrt{3}}{2}.