Triangle with two fixed vertices and the third moving on a line: rate of its area

1. A.
   $\dfrac{11}{5}\,(\text{units})^2/\text{sec}$
2. B.
   $\dfrac{11}{\sqrt{5}}\,(\text{units})^2/\text{sec}$
3. C.
   $\dfrac{13}{\sqrt{5}}\,(\text{units})^2/\text{sec}$
4. D.
   $\dfrac{13}{5}\,(\text{units})^2/\text{sec}$

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1. A.
   $a + b$
2. B.
   $-\left(\dfrac{a + b}{2}\right)$
3. C.
   $\dfrac{a - b}{2}$
4. D.
   $\dfrac{a}{2}$

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