Answer:
Ok, we know that the width of the door is 4.20m.
Then, if the arc is at the top of the door, we will have that the diameter of that "half circle" will be equal to the width of the door.
Then the diameter of the arc is 4.20m
And the radius is defined as half of the diameter, so we have:
radius = 4.20m/2 = 2.10m
And we can write this in cm (because the probelm ask for it)
1m = 100cm
then 2.10m = 2.1*100cm = 210cm
Now, we know that the perimeter of a circle is:
P = 2*pi*r
where pi = 3.14
Then the perimeter (or arc) of half a circle will be half of that:
Arc = (2*pi*r)/2 = pi*r
and r = 210cm, pi = 3.14
Arc = 3.14*210cm = 659.4cm
