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Geometrically Finding the Area of a Trapezoid

I hope the picture says it all.  The last step uses the fact that the area of a parallelogram of base b and height h is simply bh, which can easily be proved geometrically by slicing off a triangle and sliding it to the other side to form a rectangle with the same base and height.

Sorry for the lack of posts recently, but they should perk up as the school year begins and I have lots of interesting things to think about all the time.