Question

Could someone please help me with number 20?

Answer 1

First, pretend the semicircle isn't cut out of the triangle and find the area of that equilateral triangle using the equation:
$a = \frac{ \sqrt{3} }{4} {l}^{2}$
where l=the length of one side of the triangle (2+2+4=8mm).

Then find the area of the semicircle using the equation for the area of a circle:
$a = \pi {r}^{2}$
where r=the radius of the circle (4/2=2mm) and then dividing the area you get by 2 to get the area of the semicircle.

Finally subtract the area you got for semicircle from the area of the entire equilateral triangle.