Australian AMC 2019 - Intermediate Question 18

Question 18

The area of the shaded triangle inside this rectangle is

1. $\frac12(x+y)^2$
2. $x(x+y)$
3. $y(x+y)$
4. $\frac12(y^2-x^2)$
5. $\frac12(x^2+y^2)$