why do we use $\pi \cdot d/2+d$ for semicircles I'm in grade seven and right now we are learning perimeter of a semicircle, and I noticed we use the formula $\pi\cdot D/2+D$ (where $D$ is the diameter) instead of $(\pi+2)D/2$, why do we do the first formula instead of the second, where they both have the same answer?
You can use either formula as they are identical, as you've noted.  A reason for preferring the first is that it makes the perimeter calculation explicit: $\pi\cdot D/2$ is the length of the curve, and $+D$ then adds on the straight line across the bottom.
The results are the same:
$$\left( \pi + 2 \right) D/2 = \pi D/2 + D.$$
The perimeter of a circle of radius $r$ is $2 \pi r$. Since the diameter, $D$, is twice the radius, $D = 2 r$ this gives the perimeter of a circle as $\pi D$. It follows that the perimeter for a semi (half) circle is $\pi D/2$. Now just add the remaining length $D$ of the line joining the two ends of the semi circle.

Since the results are the same which one you use is simply your preference. Personally, I prefer the second form since it makes clear the contributions from each part of the curve (the semi-circle versus the straight line segment).