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Why parallelogram area is |ad-bc|?

Here is my question.

The area of parallelogram is the difference of these two rectangles (red rectangle - blue rectangle). This is not intuitive for me. If you also think it is not so intuitive, you might interested in my slides. I try to explain this for hight school students.

Slides: A bit intuitive (for me) explanation of area of parallelogram (to my site, external link)


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