#### Answer

$K=\sqrt{s(s-a)(s-b)(s-c)}$, where $s=\dfrac{a+b+c}{2}$

#### Work Step by Step

According to Heron's formula, the area of a triangle with sides $a,b$ and $c$ is given by:
$K=\sqrt{s(s-a)(s-b)(s-c)}$
where $s=\dfrac{a+b+c}{2}$ and $K$ is the area of the triangle.
Therefore, the area of a triangle when the lengths of the three sides are known can be calculated with the above formula.