The area of triangle is
[tex] A_{triangle}=\dfrac{1}{2}\cdot \text{base}\cdot \text{height}. [/tex]
From the diagram it is seen that
[tex] \text{ base }=r,\\\text{ height }=x. [/tex]
Then
[tex] A_{triangle}=\dfrac{1}{2}\cdot r\cdot x. [/tex]
Answer: correct choice is D.
