#### Answer

$a(b)=\frac{\sqrt 3}{4}b^2$
a - area
b - base

#### Work Step by Step

We'll use $a$ for the area, $b$ for the base and $h$ for the height.
Because of the properties of an equilateral triangle, we know that $h$ intersects the base in the middle. This means $h^2+(\frac{b}{2})^2=b^2$. From that we get $h=\frac{\sqrt 3}{2}b$.
Then, we obtain: $a=\frac{1}{2}bh=\frac{1}{2}b\frac{\sqrt 3}{2}b=\frac{\sqrt 3}{4}b^2$