## Precalculus: Concepts Through Functions, A Unit Circle Approach to Trigonometry (3rd Edition)

$A=\frac{\sqrt 3}{4} x^2$
Since the area $A$ of an equilateral triangle is $\frac{\sqrt 3}{4}$ times the square of the length $x$ of one side, then we have $$A=\frac{\sqrt 3}{4} x^2 .$$