First, calculate the area of two circles. Subtract the area of the big circle to get the area of the blank part, and then subtract the area of the blank part to get the area of the shadow part.

For example:10 *10 * 3.14 = 78.5 square decimeter, the area of a quarter circle is10 *12 = 50 square decimeter, and the area of a triangle is 78.2.

Formula derivation

Perimeter (c): the diameter (d) of a circle. If the circumference (c) of a circle is divided by the diameter (d) of the circle, it is equal to π times the diameter (d) of the circle.

C = π D. The diameter (d) of the same circle is twice the radius (r) of the circle, so the circumference (c) of the circle is equal to 2 times π times the radius (r) of the circle, and c = 2 π R. If the circle is divided into several parts, an approximate rectangle can be made.

The formula of the area of a circle is: pi times the square of radius, which can be expressed in letters as: S=πr? Or S=π*(d/2)? . (s represents the area of a circle, π represents pi 3. 14 15926 ..., r represents radius, and d represents diameter)?

Formula derivation: the circumference (c) of a circle: the diameter (d) of a circle, and the circumference (c) divided by the diameter (d) of a circle equals π, so the meaning of multiplication equals π times the diameter of a circle (d) equals the circumference (c) of a circle, and c = π D. The diameter (d) of the same circle is twice the radius (r) of a circle, so the circle.

It is equal to 2 times π times the radius (r) of the circle, and c = 2 π r. If the circle is divided into several parts, an approximate rectangle can be made. The width of the rectangle is equal to the radius (r) of the circle, and the length of the rectangle is half the circumference (c) of the circle. The area of a rectangle is ab, and the area of a circle is: the square of the radius (r) of the circle times π, and S=πr? . ?

1, a circle, how many times the radius is enlarged or reduced, the diameter and circumference are also enlarged or reduced by the same factor. The multiple of area expansion or contraction is the square of this multiple. ?

2. When the perimeters of rectangle, square and circle are equal, the area of circle is the largest, the square is in the middle, and the area of rectangle is the smallest. On the contrary, when the area is the same, the circumference of a rectangle is the longest, the square is in the middle and the circumference of a circle is the shortest.