Q:

formula for finding perimeter of half circle

i was looking for this problem
http://www.majortests.com/gre/problem_solving_expl.php?exp=50313039243130243330
and was surprised if it is correct,we know that  circumference or perimeter of  circle is equal to
$C=\pi*D=2*\pi*R$
but what about perimeter of half circle?i found following link
http://www.mathvillage.info/node/159
which says that perimeter of half circle is 
$c=\pi*D/2+D$
where $D$ is diameter of course,but in problem,link of  which i have posted,it is not considered and it said that perimeter of half circle is just  $\pi*D/2$,in case of this test come on GRE exam,i should now of course which formula is correct,so please help me to determine there is  error made in formula  $\pi*D/2+D$ or what? thanks in advance

A:

In the linked question, we are just interested in the upper half of the circle and not at all the base (diameter). So, that is why we are just using the perimeter as $\pi*r$.
Every formula is used in certain context. And understanding context is more important than the formula.
ADDED- Perimeter can be considered as the length of a tape required to cover the given figure. So, now i guess you can see which formula is to be used.

