The straight edge of the semicircle is 4 units, as this is this the diameter.
The circumference of the full circle is
C = pi*d = pi*4 = 4pi
which is the distance around the full circle (aka circle's perimeter)
So half of this is 4pi/2 = 2pi
Add this onto the straight edge length to get 4+2pi as the exact distance around the entire semicircle. This includes both straight and curved portions.
If you use the approximation pi = 3.14, then 4+2*pi = 4+2*3.14 = 10.28 is the approximate answer. To get a more accurate answer, use more decimal digits in pi.
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In summary,
exact answer in terms of pi is 4+2pi units
approximate answer is roughly 10.28 units (using pi = 3.14)
