1. If f (x) = {2x (x > 0) - x (x ≤ 0), then f (f (- 3))=

1. If f (x) = {2x (x > 0) - x (x ≤ 0), then f (f (- 3))=

-30
So f [f (- 3)]
=f(3)
=2×3
=6

What is the temperature at which gaseous oxygen becomes liquid oxygen

At room temperature and normal pressure, oxygen is gaseous. But at normal pressure, when the temperature drops to - 183 ℃, oxygen becomes light blue liquid, and when the temperature continues to drop to - 218 ℃, it becomes light blue solid crystal

If XA = 3, XB = 5, then x3a-2b=______ ．

∵ XA = 3, XB = 5, ∵ x3a-2b = (XA) 3 ÷ (XB) 2, = 33 ÷ 52, = 2725

How to solve the problem of surface area and volume

It depends on the grade
For example, below high school, is a few formulas
If it's a university, use calculus, type 1 curvilinear integral, type 2 curvilinear integral, Gauss theorem, Stokes equation and so on

For example, the speed of cheetah is 40m / s, the speed of brown petrel is 5km / min, and the speed of sailfish is 108km / h

Cheetah 40m / S
Brown petrel 5000 / 60 = 83.33m/s
Sailfish 108 / 3.6 = 30m / S
The answer is the brown petrel. Come on

Now you can count the PI to a few decimal places

With the help of large-scale computers, 4.2 billion decimal places have been calculated in 1995. So to speak, as long as you are patient, computers can theoretically calculate any number of decimal places. If you are interested, you can download a program called "superpi" and play with it on your own computer

The tangent plane of ellipsoid x ^ 2 + y ^ 2 / 4 + Z ^ 2 / 4 = 1 is made in the first hexagram to minimize the volume of tetrahedron surrounded by three coordinate planes,
The tangent plane of ellipsoid x ^ 2 + y ^ 2 / 4 + Z ^ 2 / 4 = 1 is made in the first hexagram to minimize the volume of the tetrahedron surrounded by three coordinate planes. The tangent point coordinates and the minimum volume are obtained

Factorization x4-3x-4

【1】 The original formula = [(x ^ 4) + X & # 179;] - [x & # 179; + X & # 178;] + [x & # 178; + x] - [4x + 4] = x & # 179; (x + 1) - X & # 178; (x + 1) + X (x + 1) - 4 (x + 1) = (x + 1) (X & # 179; - X & # 178; + x-4)

Five digit a329b can be divided by 72. What numbers do a and B represent

72 is 8 × 9, so a329b must be a multiple of 8 and 9
First of all, look at the multiple of 8, because the whole thousand must be a multiple of 8, so 29b is required to be a multiple of 8, and B = 6
If we look at it again, it must be a multiple of 9, that is, a + 3 + 2 + 9 + 6 is a multiple of 9. We get a = 7
So the number is 73296

It is known that F1 and F2 are the left and right focus of the ellipse x ^ 2 / 100 + y ^ 2 / b ^ 2 = 1 (0 < B < 10), and P is the point on the ellipse. Find the maximum value of | Pf1 | * | PF2 |
|PF1|*|PF2|≤(|PF1|+|PF2|)^2/4=4a^2/4=a^2=100
——Why?

You did it
x+y≥2√(xy)
x. Y is a positive number
If and only if x = y, x + y takes the maximum, otherwise XY has the minimum
This question
a=10
2a=20
PF1+PF2≥2√(PF1*PF2)
√(PF1*PF2)≤(PF1+PF2)/2
PF1*PF2≤(PF1+PF2)²/4=(2a)²/4=a²=100
That's how it came about
And you can do it with quadratic functions
Let Pf1 = X
Then Pf1 * PF2 = x (20-x) = - X & sup2; + 20x = - (X-10) & sup2; + 100
0