A shaded shape has two semicircles. One has a diameter of 20cm other 30cm.
Whats the perimeter of the shaded shape?
The circumference of a circle is $\displaystyle \pi d$ where $\displaystyle d$ is the diameter. You want the perimeter of the semicircle which will be half of the circle. So you will need half the circumference plus the diameter (as if you only have half the circumfence then you will calculate the arc and have the diameter left off) hence $\displaystyle \frac{\pi d}{2} + d$.
Will you be able to continue or do you want to expand?
To the OP :
Note that the perimeter doesn't change if :
- the white circle is not centered (3 and 7 aside for example)
- the white circle is on the opposit side
If you interprete it like Air did, that is to say 2 semicircles that are not linked, the perimeter is sure to be different.
So tell us what the shaped shape is