The diameter of the small circle is 2 / 5 less than the radius of the big circle, the radius of the small circle is 3cm, the area of the big circle is? The perimeter ratio of the small circle to the big circle is? The area ratio is?

The diameter of the small circle is 2 / 5 less than the radius of the big circle, the radius of the small circle is 3cm, the area of the big circle is? The perimeter ratio of the small circle to the big circle is? The area ratio is?

The radius of the great circle is 3 (1-2 / 5) = 5cm
So the area is 3.14 × 5 × 5 = 78.5 square centimeters
Perimeter ratio = (1-2 / 5): 1 = 3:5
The area ratio is (3 × 3): (5 × 5) = 9:25

Primary school sixth grade mathematics volume I on the circumference and radius of a circle, urgent

The relationship between circumference and radius of a circle is as follows:
Perimeter = radius multiplied by 2
Radius = perimeter divided by 2
Perimeter divided by radius = 2
I'm also in grade six, but we are frind next semester!

(1) There is a big clock in the hall of the hotel. Its minute hand is 48 cm long. How many cm does the tip of the minute hand travel when it rotates for a week?
(2) School playground (as shown in the figure on the right, unit: m), what is the perimeter of the playground? What is the area of square meters? (3) a circular iron ring, with a diameter of 40 cm, how many iron bars do you need to make such an iron ring?
(4) There is a round goldfish pond with a diameter of 15 meters in the children's Park. How many steel bars should be used to make two circles of round railings around the goldfish pond? (5) the sand pile on the ground covers an area of 15.7 meters, so how many meters is the diameter of the sand pile? (6) the outer diameter of a bicycle tire is 70 cm, if it turns 120 cycles per minute, How many kilometers can you travel in an hour? (keep the whole kilometer number) (7) one iron ring has a diameter of 60 cm, and it rolls from the east end of the playground to the west end for 90 turns. The other iron ring has a diameter of 40 cm. How many turns does it have to roll from the east end to the west end?
(8) The outer diameter of a kind of automobile tire is 1.02 meters. It turns 50 cycles per minute. How many meters does the wheel advance per minute?
(9) The radius of a bicycle wheel is 40 cm. The wheel turns 100 times per minute. How many minutes does it take to cross the 2512 meter bridge? (10) the length of the hour hand of a big clock is 30 cm, and the length of the minute hand is 40 cm. What is the distance between the hour hand and the point of the minute hand? (11) the circumference of a semicircle is 15.42 decimeters, and what is the area of the semicircle?

1、3.14×48×2,
=3.14×96,
=44 (CM)
A: the distance is 301.44 cm
2. The perimeter of the playground is 3.14 × 60 + 100 × 2,
=188.4+200,
=388.4 (m);
A: the perimeter of the playground is 388.4 meters
3. 3.14 × 40 = 125.6 (CM)
A: to make such an iron ring, you need 125.6cm iron bars
4. 14 × 15 × 2 = 94.2 (m),
A: at least 94.2m steel bars should be used
5. 15.7 △ 3.14 = 5 (m),
A: the diameter of sand pile is 5 meters
6. It is known that d = 70 cm,
C=πd
14 × 70 = 219.8 (CM);
One minute: 219.8 × 120 = 26376 (CM);
26376cm = 0.26376km;
It can work in one hour: 0.26376 × 60 ≈ 16 (km);
A: 16 kilometers an hour
7、（3.14×60×90）÷（3.14×40）
=16956÷125.6,
=135 (circle),
A: it takes 135 turns for a 40 cm diameter ring to roll from the east end to the west end
8、1.02×3.14×50,
=3.2028×50,
=14 (m);
A: the wheels advance 160.14 meters per minute
9. 2 × 3.14 × 40 = 251.2 (CM);
2 × 100 = 25120 (CM);
25120 cm = 251.2 m;
2 = 10 (minutes);
A: it takes about 10 minutes to cross the 2512m bridge
10、3.14×30×2×2
=3.14×120,
=8 (CM);
3.14×40×2×24
=3.14×1920,
=6028.8 (CM)
A: the distance between the point of the hour hand and the minute hand is 376.8 cm and 6028.8 cm respectively
11. Let its radius be r decimeter,
2πr /2 +2r=15.42,
πr+2r=15.42,
3.14r+2r=15.42,
5.14r=15.42,
5.14r÷5.14=15.42÷5.14,
r=3；
Each area of the semicircle is;
3.14×3^2÷2,
=3.14×9÷2,
=28.26÷2,
=13 (square decimeter);
A: the area of this semicircle is 14.13 square decimeters

If the circumference of the bottom surface of a cone is 4 π and the angle between the axis and the generatrix is 30 °, the area of the cross section of the cone axis is 0
Need a solution

∵ given C = 4 π, the radius of the bottom surface r = 2
The angle between the axis and the bus is 30 degrees
Bus length L = 4
Shaft length = 2 √ 3
Therefore, the area of the cross section of the cone axis = ½ × 2R × 2 √ 3
=½×4×2√3
=4√3

The figure below is the expansion of a cone to find the degree of the central angle of the sector
(the radius of a circle is 15.)
(for one side of the sector, let's assume that the radius of the whole circle is 45.)
I didn't say that!
The picture is two parts, circle and sector!
(the radius of a circle is 15.)
(for one side of the sector, let's assume that the radius of the whole circle is 45.)

Oh, I see what you mean
Good solution
That is, divide the circumference of the small circle by the circumference of the large circle you assume, and multiply it by 360. The result is the center angle of the sector
（2πr/2πR）*360=120°
But the guy upstairs didn't do anything wrong

If the generatrix length of a cone is three times the radius of its bottom circle, then the center angle of its side expanded view is______ Degree

2 π r = n π· 3r180, the solution is n = 120 ° and the center angle of the side view is 120 degrees

If the base radius of a cone is 3 and the generatrix length is 5, then the center angle of its side expanded view is equal to ()
A. 108°B. 144°C. 180°D. 216°

The circumference of the bottom surface of the cone is 2 π × 3 = 6 π. Let the center angle of the side expanded view be n, n π × 5180 = 6 π, and the solution is n = 216

The radius of the bottom of the cone is 40 cm and the length of the generatrix is 90 cm

Let the angle of the center of the expanded side view of the cone be N.2 π × 40 = n × π × 90180, and the solution is n = 160. The side area of the cone is π × 40 × 90 = 3600 π cm2

We know that the height of the cone is 3 √ 3, and the side view is semicircle. Find the ratio of the generatrix length to the bottom radius of the cone

The height of the cone is 3 √ 3, and the side expanded view is a semicircle. Find the ratio of the generatrix length to the bottom radius of the cone. Suppose the bottom radius of the cone is r, and the generatrix length is R. R is the radius of the semicircle formed by the side expanded view. From the perimeter formula, 2pi * r = pi * r, so the ratio of the generatrix length to the bottom semicircle of the cone is 2:1

Take a disc with a radius of 8 cm, cut out a sector with a center angle of 90 degrees, and make the remaining part into the side of a cone. Then the height of the cone is
fast

R = 8; center angle = 360-90 = 270 degrees = > sector arc length = bottom circumference of cone = 2 * pie * r * 270 / 360 = 12 pie
The base radius of cone r = 12 Pie / 2 pie = 6
High H = under radical (8 + 6) * (8-6) = under radical 28 = twice radical 7