About regular heptagons, the side lengths of which are proportional to the numbers 5 and 4
About regular heptagons, the side lengths of which are proportional to the numbers 5 and 4, are described in a circle. The radius of one of them is 3 cm larger than the radius of the other. Calculate the lengths of the diameters of the given circles
The radius of a circle circumscribed about a regular N-gon can be calculated by the formula
ρ = a / (2 * sin (180 ° / N),
where a is the length of one side of the N-gon.
Let the lengths of the sides of the given heptagons be 5x and 4x, respectively. Then the radii of the circles described around them are equal, respectively
R = 5x / (2 * sin (180 ° / 7);
r = 4x / (2 * sin (180 ° / 7),
and the ratio of these radii is
R / r = 5/4,
or
R = 1.25 * r.
Since by condition R = r + 3 cm, then
r + 3 = 1.25 * r,
where
0.25 * r = 3;
r = 12 cm – the radius of the small circle;
R = 1.25 * r = 1.25 * 12 = 15 cm – the radius of the great circle.
Then the diameters of these circles are equal respectively
d = 2 * r = 2 * 12 = 24 cm;
D = 2 * R = 2 * 15 = 30 cm.