Suppose the length of the small rectangle is x and the width is y, then xy=1.

The area of ??the shaded part = the area of ??the shadow of the triangle in the first vertical rectangle + the area of ??the shadow of the trapezoid in the second vertical rectangle + the area of ??the shadow of the triangle in the last two series.

=2xy×12+(2x+3x)y×12+3x×2y×12

=132xy=6.5xy=6.5．

So the answer is: 6.5.