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Method B2

Here we lower point A so that the invariant line has a gradient of 1.
What will happen to the position of C' and to the shape of the green image? You might find that the shape is quite hard to visualise.

What should happen to the area of the green image?
Can you show that this really is true for the resulting shape?

Next we lower A by another unit, so the invariant line has a gradient of 2.