AB and CD are parallel. [br]E, F are midpoints of 2 sides.[br][br]Measure the lengths of the segments AB, CD and EF.[br]How are they related?

The silent video below shows the construction of this trapezium. [br][list][*]A line parallel to AB is drawn and hidden during the construction. [/*][*]Use the measure tool to find the lengths of some segments (or simply show the 'value' of a segment object from the style bar)[/*][*]Open the algebra view on the left. Input expressions about some lengths to check their relation.[/*][/list]