Radius of circle (r)

Diameter of circle (d)

Circumference of the circle (c)

Area of a circle

2 decimeter

8 cm

62.8 meters

2. Fill in the blanks.

The radius, diameter, perimeter and area of (1) circle have been enlarged by 3 times, () times and () times respectively.

(2) A wire just surrounds a circle with a radius of 2 decimeters. If it is changed to a square, the side length of the square is () cm and the area is () cm 2.

(3) Seiko brand quartz clock has an hour hand length of 8 cm, a minute hand length of 10 cm, the tip of the hour hand goes () cm, and the tip of the minute hand goes () m all day.

(4) Draw a maximum circle in a square with a circumference of 32 cm, and the area of this circle is () square cm.

(5) Both a circle and a semicircle are axisymmetric figures, and the circle has an axis of symmetry () and the semicircle has an axis of symmetry ().

judge

(1) For two circles, the circumference of the circle with long radius must be long. ( )

(2) If the perimeters of two circles are equal, then their areas must be equal. ( )

(3) The circumference of a semicircle is half that of a circle. ( )

(4) A circle with a radius of 4 cm has a greater pi than a circle with a radius of 3 cm. ( )

Step 4 solve practical problems

(1) circular pool with a diameter of 8m. How many square meters does it cover? How many meters does it take to walk around the swimming pool?

(2) Cut out the largest semicircle in a rectangle with a length of 50 cm and a width of 20 cm, and find the area of this semicircle.