The perimeter difference between the two circles is 12 cm. It is known that the ratio of the perimeter of the small circle to that of the large circle is 25:81, so what is the area of the small circle

The perimeter difference between the two circles is 12 cm. It is known that the ratio of the perimeter of the small circle to that of the large circle is 25:81, so what is the area of the small circle

2.284cm*2
R / r = 25 / 81 2 π R-2 π r = 12 two equations and two unknowns can calculate r = 75 / 28 π, and then according to the area formula s = π R * 2, s = 2.284cm * 2 can be obtained

The circumference of a semicircle is 15.42 cm. What is the area of the semicircle in square centimeter? And why,
The point is to explain why

Semicircle arc length + diameter = semicircle perimeter, namely 3.14 * r + 2 * r = 15.42, r = 3, semicircle area = 0.5 * 3.14 * 3 * 3 = 14.13