An equilateral triangle PQR of side X.

有幾個方法

= 1/2 BC sin A

= 1/2 (x) (x) sin (60)

=1/2 (√3/2) x^2

=(√3/4) x^2

有個煩d 既方法, 講埋你聽

By heron's formula

三角形Area= √s √(s-a) √(s-b) √(s-c) a,b,c 係三條邊 s=1/2(a+b+c)

=√s x √(s-X)^3 三條邊一樣

=√(3X/2) x √(3X/2-X)^3

=√(3X/2) x √X^3

= √3X^4/ 4

=(√3/4)X^2

The hight of the triangle = √[x^2 - (x/2)^2] = (x√3)/2
Area = { (x√3)/2 * x/2 } = x^2(√3/4)

2006-12-31 16:35:44 補充：
The hight of the triangle = √[x^2 - (x/2)^2] = (x√3)/2 - Find the hight of the triangle using Pythagoras Theorem. Method introduced by the user above is quicker, but if you haven't learnt that, just use the Pythagoras Theorem. It's easy to remember.