3. Let $a, b, c, d$ be positive real numbers, $a < b$ and $c < d$. There is a triangle with side lengths $\sqrt{a^{2}+c^{2}}, \sqrt{b^{2}+d^{2}}, \sqrt{(b-a)^{2}+(d-c)^{2}}$, then the area of this triangle is $\qquad$
Answer: $\frac{1}{2}(b c-a d)$