Area formula of polygon Irregular

The formula for calculating the inner angle of a regular polygon has nothing to do with the radius. To know that the number of sides of a regular polygon is n and the inner angle is 180 (n-2), the formula for calculating the area of an inscribed triangle with a radius of R circle: (3 times the root sign 3) divide by 4 and multiply by the area of an circumscribed triangle with a radius of R Square: 3 times the root sign 3 R circumscribed Square: 4R inscribed Square: 2R square pentagonal or more

Solving a problem with the equivalent substitution method in Physics
Use two dry batteries, three switches, resistance to be measured, [voltmeter (0-3v, 0-15v), ammeter (0-0.6a, 0-3A), sliding rheostat (100 Ω 2a), rheostat box (0-9999 Ω 2a)] and conductor to measure a resistance of about 1000 Ω. Note: only three of them can be selected

Choose voltmeter, sliding rheostat, rheostat box, or use ammeter instead of voltmeter
First, connect the battery switch, sliding rheostat and the resistance to be measured in series. Connect the voltmeter to both ends of the resistance to be measured. Adjust the sliding rheostat to make the voltmeter indicate a fixed value. Then, connect the resistance box into the circuit instead of the resistance to be measured. Adjust the resistance value of the resistance box to make the indication of the voltmeter the same as before. Record the resistance value of the resistance box at this time. Repeat for three times, The voltmeter takes three different fixed values. The average value of the measured resistance of the transformer box is the resistance value to be measured
If we use ammeter instead of voltmeter, we only need to string ammeter into the circuit

The formula of universal gravitation and Kepler's third law
In Kepler's third law, there is a ^ 3 / T ^ 2 = K
So is k a constant? Does it have a specific value?
According to the textbook, the earth planet is attracted by the sun f = 4 π ^ 2K * m / R ^ 2
But it doesn't seem to have anything to do with the mass of the sun (4 π ^ 2K is constant). What's going on?
(the textbook is compulsory 2 of the people's education press. It seems that the proof of gravity is not very rigorous.)

K is a constant for the motion of planets in the solar system around the sun
Gravity provides centripetal force: GMM / R ^ 2 = m (2 π / T) ^ 2 * r
r^3/T^2=(GM)/(4*π^2)
------Where G is the gravitational constant and M is the mass of the sun

I can't understand the meaning of the title
Use iron wire to weld a cuboid frame that is 5cm long, 4cm wide and 1cm high, and stick stickers on its five sides, at least how many square centimeters of paper is needed.
Don't count the area above or the area in front?

Don't count the largest area That is to say, don't count the area of 5 × 4

Tetrahedral p-abc three groups of opposite edges are equal respectively, and they are 25, 13 & nbsp;, 5 in turn

The three opposite edges of ∵ tetrahedron p-abc are equal, and they are 25,13 & nbsp;, 5, ∵ in turn, which can be added as a cuboid with diagonal length of 25,13 & nbsp;, 5 on three planes, and ∵ cuboid with three side lengths of 2,3,4, ∵ tetrahedron volume of 2 × 3 × 4-4 × 13 × 12 × 2 × 3 × 4 = 8

Xiao Hu and Xiao Tu calculated the product of two digits A and B. Xiao Hu misread the number a and the result was 1274; Xiao Tu misread the number a and the result was 819______ ．

When the number B is 91, the wrong number a may be 1274 ／ 91 = 14 or 819 ／ 91 = 9, because the number a is two digits, so this situation is not suitable; When the number B is 13, the wrong number a may be: 1274 ／ 13 = 98 or 819 ／ 13 = 63. Combined with the meaning of the title, the result of Xiao Hu's wrong number a is 93, so the number a is 9; Xiao Tu's wrong number a is 63, so the number a is 3; so the answer is 93

If there is a tangent perpendicular to Y-axis on the curve f (x) = AX2 + LNX, then the value range of real number a is ()
A. （-∞，0）B. （-∞，1）C. （0，+∞，）D. （1，+∞）

∵ the curve f (x) = AX2 + LNX has a tangent perpendicular to the Y axis, (x > 0) ∵ f ′ (x) = 2aX + 1x = 0 has a solution, a = − 12x2, ∵ x > 0, ∵ a = − 12x2 < 0, ∵ the value range of real number a is a < 0

Let a = {a, B, C}, B = {0,1}. How many mappings are there from a to B? Show them separately

That is to say, how many ways can we change the element of a into the element of B
For example, if a function f can change a to 0, B to 0, and C to 0, then f is a mapping from a to B
So ~ a can be changed to 0 or 1, B and C. similarly, there are six variations, namely, six maps

1、 If the inequality system X ≤ 4 x > A has no solution, then the value range of a is___ If the equations have solutions, then the range of a is_ If the equations have three integer solutions, then the range of a is_ 2、 If the solution set of linear equations x ＞ 3 x ＞ A is x ＞ 3, then the value range of a is_ 3、 If the inequality system X ≤ m, X ≥ 1 has a solution, then the range of M is_____

1、 If the inequality system X ≤ 4 x > A has no solution, then the value range of a is_______ a≥4；
If the equations have solutions, then the range of a is_______ a

A math problem, I don't understand,
x> The range of (2-x) / (1 + x) is 0
The answer is (- 1,2)

（2-x)/(1+x)=[3/(1+x)]-1
It's easy to see that 3 / (1 + x) is decreasing at (0, positive infinity)
So when it's equal to 0, the maximum is equal to 3
At positive infinity, 3 / (1 + x) tends to zero
So I got the answer

Area formula of polygon Irregular