Area of a Trapezium finding and comparing

First can you work out the formula for the area of a trapezium? Rotate out a second trapezium for a clue. Compare the shape to the formula for a parallelogram. Check your answer with the "show area formula" box. Second can you change the shape (using the 3 blue corner dots and the height dot) to make other shapes? See if you can find a rhombus, parallelogram, rectangle, square and any tyoe of triangle. How does the formula change for the different shapes? Look at the values for a and b on the shape and in the formula. Use the rotate slider to see that it's still always a parallelogram.