Under certain conditions, a certain cell divides into two after one hour. It is known that a certain number of cells after 20 hours of division, the number of cells becomes to the 30th power of 2, then the number of cells to the 15th power of 2 needs: () A. 10 hours B, 15 hours C, 5 hours D, 19 hours

Under certain conditions, a certain cell divides into two after one hour. It is known that a certain number of cells after 20 hours of division, the number of cells becomes to the 30th power of 2, then the number of cells to the 15th power of 2 needs: () A. 10 hours B, 15 hours C, 5 hours D, 19 hours

The correct answer is C, 5 hours!
Suppose it is a cell, then after 20 hours, the number of cells should be the 20th power of 2, but actually it is the 30th power of 2, so the original number of cells should be the 10th power of 2! Now it only takes 15-10 = 5 hours to get to the 15th power of 2

1. If someone writes such two results: when x = 79.08, y = 21.57, x ^ 2 + y ^ 2-2xy = - 490.86, x ^ 2 + y ^ 2 + 2XY = 9572.81. Without calculation, can you quickly judge whether the results are correct? Why?
2. If x ^ 2 + KX + 4 is the square of an integer, then the value of constant k is ()
A.+1,-1 B.+2,-2 C.+4,-4 D.+8,-8

The first question is wrong
adopt
X ^ 2 + y ^ 2 + 2XY = 9572.81
X + y square root = about 100
That is, x + y = about 10
In the meaning of the title, x + y = 100 or so
So wrong
C for question 2

In the "I love my motherland" speech contest, a total of 15 participants want to know whether they can enter the top 8, only need to know their own results and the ()
A. Mean B. mode C. median D. variance

As there are 15 players in total, the eighth player's score is the median. To judge whether he is in the top eight, he should know his score and median

A right angled trapezoid has an upper bottom of 6 decimeters. When the upper bottom is increased by 2 decimeters, it becomes a square. The area of the original trapezoid is______ ．

(6 + 6 + 2) × (6 + 2) △ 2 = 14 × 8 △ 2 = 56 (square decimeter) answer: the area of the original trapezoid is 56 square decimeter

Every step of the process is needed to solve binary linear equations, 3 / 7X = 3Y + 90x (1-3 / 7) = 5Y

3/7x=3y+90 3/7x-3y=90 1/7x-y=30
X (1-3 / 7) = 5Y = = = > 4 / 7x-5y = 0 = = = > 1 / 7x-5 / 4Y = 0 = = > 1 / 4Y = 30, y = 120, x = 1050

If the sum of the first four terms of the arithmetic sequence {an} is 40, the sum of the last four terms is 80, and the sum of all terms is 210, then the number of terms n is ()
A. 12B. 14C. 15D. 16

According to the meaning of the title, a1 + A2 + a3 + A4 = 40, ① an + an-1 + An-2 + an-3 = 80, ② from the nature of the arithmetic sequence, we can know that ① + ② can be obtained, 4 (a1 + an) = 120 {(a1 + an) = 30 can be obtained from the first n terms and formula of the arithmetic sequence, Sn = n (a1 + an) 2 = & nbsp; 15N = 210, so n = 14, so B is selected

Absolute value limit problem: if LIM (x - > 0) f (x) = a is known, then LIM (x - > 0) | f (x) | = | a |? If so, please prove it with the definition of limit. If not, please give a counter example,

This conclusion is correct. The key to proof is to use the absolute value inequality |||||||- ||||||≤||||a-b |
Because LIM (x - > 0) f (x) = a, according to the definition of function limit, we can know that for any ε, there exists δ to make | x appropriate|

Known wind, air volume, how to choose how much power generator, consult the formula expression, thank you!

Wind power = 0.5pav ^ 3
Wind energy conversion limit efficiency: 0.593
Theoretical generator power = 0.593 * 0.5pav ^ 3
In fact, there are more losses in the process of wind energy conversion. In addition, certain coefficients should be reserved for generator selection
p: Air density
A: Swept area
v: Wind speed

The total capacity of the Three Gorges Reservoir on the Yangtze River is about three times that of the Xiaolangdi Reservoir on the Yellow River. The total capacity of the Xiaolangdi Reservoir on the Yellow River is 26 billion cubic meters less than that of the Three Gorges Reservoir on the Yangtze River
What is the total capacity of Xiaolangdi Reservoir on the Yellow River and the Three Gorges Reservoir on the Yangtze River?
The solution of a series of equations

Let's solve the linear equation of one variable
Suppose the total storage capacity of Xiaolangdi Reservoir on the Yellow River is x billion cubic meters, because the total storage capacity of the Three Gorges Reservoir on the Yangtze River is about three times that of Xiaolangdi Reservoir on the Yellow River, so the storage capacity of the Three Gorges Reservoir on the Yangtze River is 3x billion cubic meters. We also know that the total storage capacity of Xiaolangdi Reservoir on the Yellow River is 26 billion cubic meters less than that of the Three Gorges Reservoir on the Yangtze River, that is to say
3x-x=260
2x=260
x=130
Therefore, the capacity of Xiaolangdi reservoir is 13 billion cubic meters, and that of the Three Gorges reservoir is 130x3 = 39 billion cubic meters

In the Taylor formula of order n (n > 2) in which the function f (x) is expanded by the power of (x-1), the coefficient of the term (x-1) ^ 2 is
Like the title,

That's f "(1) / 2!
That is f "(1) / 2