What is the area of the triangle with vertices (0, 0), (4, 1) and (2, 5)?

With one vertex at the origin the shoelace formula reads \frac{|x_{1}y_{2} - y_{1}x_{2}|}{2} = \frac{|4 \times 5 - 1 \times 2|}{2} = \frac{18}{2} = 9. Forgetting the halving gives 18.