A round fountain with a radius of 20 meters. Now we need to build a 1-meter-wide path around the fountain. The path covers an area of 20 meters

A round fountain with a radius of 20 meters. Now we need to build a 1-meter-wide path around the fountain. The path covers an area of 20 meters

The total radius is 20 + 1 = 21 meters
So the area of the path is 3.14 × 21 × 21-3.14 × 20 × 20 = 128.74 square meters

3, - 5, - 11, 7 these four numbers calculate 24 points, need a method

（7-3）*{（-5 ）- （-11）}

Derivation y = [arccos (2 / x)]'and y = [ln (secx + TaNx)] '

The reciprocal of y = arccosx is y = - 1 / radical 1-x ^ 2
therefore
The derivative of y = [arccos (2 / x)] is
Y '= 2 / x ^ 2 / (radical 1-4 / x ^ 2) = 2 / radical x ^ 4-4x ^ 2
2.y'=[1/(secx+tanx)]*(secxtanx+(secx)^2)
=secx

In physics, ρ is density, M is mass, and V is volume. Therefore, the formula of density can be expressed as ρ = m / v（

In physics, ρ is density, M is mass and V is volume, so the formula of density can be expressed as ρ = m ／ v,
What is this? What are the options?

What is the surface area and volume of 8cm long, 5cm wide and 4cm high

The surface area is 2 (8 * 5 + 8 * 4 + 5 * 4) = 184, and the volume is 8 * 5 * 4 = 160

Point P (x, y) is a point in the plane rectangular coordinate system. If XY is greater than 0, then the position of point P is___

First quadrant or third quadrant

Build a cement road with a width of 3.6 meters and a thickness of 20 cm. If 7.2 cubic meters of concrete is mixed, how many meters can it be paved?

20 cm = 0.2 m, 7.2 △ 3.6 × 0.2, = 7.2 △ 0.72, = 10 m; a: it can be paved for 10 m

a> B > C > 0, what is the minimum value of 2A square + 1 / AB + 1 / a (a-b) - 10ac + 25C square?

The minimum is 4
The reason is as follows: B (a-b) = a ^ 2 + 4 / A ^ 2 + (a-5c) ^ 2
>=4
If and only if B = A-B, a = 5C, a ^ = 2, the equal sign holds

21000 microns is equal to? Meters, and 80mm is equal to? Nanometers

Two solid balls made of iron and aluminum were used to compare their mass and density

In the same volume, the mass of iron ball is larger than that of aluminum ball
With the same mass, the volume of aluminum ball is larger than that of iron ball
The density of iron is higher than that of aluminum