Deriving the Area of Trapezoid

Given below is a trapezoid with bases b₁ and b₂ and height h. Move the slider to the extreme right and observe what happens.

Questions

When alpha is 180 degrees, what shape is formed? Justify your answer.
How is the area of the shape in 1 computed based on the figure?
What is the relationship between the area of the shape in 1 and the area of the trapezoid?
What formula will describe the area of the trapezoid based on the given above?