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There are 10 students in a group, including 3 girls. Two representatives are elected. The probability of at least one girl being elected is () A. 715B. 815C. 35D. 25
The total number of basic events is C102 = 45. There are at least one female student selected by the topic meaning knowledge, including two situations: one is that there is one female and one male student, and the other is that there are two females. When there is one female and one male student, there is a total of C31 · C71 = 21. When there are two females, there is a total of C32 = 3. The number of basic events included in the event "at least one female student is elected" is 21 + 3 = 24. The probability of "at least one female student is elected" is 2445 = 815 Choose B
A batch of products were randomly selected, including 10 pieces, 8 pieces of authentic products and 2 pieces of defective products. One piece was selected from them each time, and the returned one was selected three times. What is the probability that exactly 2 pieces are authentic products?
Let event a denote that there are exactly two genuine products, and the returned products are extracted three times, then one of the 10 products is selected each time, P (a) = C (1,3) × 8 × 8 × 2 / 10 × 10 = 0.384
C (1,3) denotes the order of defective products and genuine products
Mathematical probability problem of compulsory 3 in senior one
P(AUB)=P(A)+P(B）=1
A and B are mutually exclusive and opposite. Why?
Just because a and B are irrelevant, does the above formula only have operational significance?
If P (AUB) = P (a) + P (b) = 1
A and B may not be opposite.
Mutually exclusive events are two events that cannot happen at the same time. P (a + b) = P (a) + P (b)
Opposite event means that one of two mutually exclusive events must occur. P (a -) = 1-p (a)
So from P (a) + P (b) = 1, we can see that a and B are opposite events [because sum is 1]
But not necessarily mutually exclusive [because P (a ∪ b) may not equal P (a + b)]
What is the acceleration of a person falling from a 100 meter free fall?
Specific calculation method and formula explanation-
On the surface of the earth {100m, the radius of the earth is negligible}, gravity provides centripetal force, while the centripetal force required for the earth's rotation is relatively small {in contrast, gravity.}. The difference between gravity and centripetal force is gravity, and then divided by M is acceleration. In the calculation process, the difference between acceleration provided by Wang You gravity and centripetal acceleration can also be directly used, Don't help me
The answers to the unit test questions (10) and the unit test questions (9) of the seventh grade mathematics (Beijing Normal University Edition)
Unit test questions of Mathematics for Grade 7 (Part 2 of Beijing Normal University Edition) (10)
(the second parallel line and intersection line)
Unit test questions of Mathematics for Grade 7 (Part 2 of Beijing Normal University Edition) (9)
(operation of integral)
There are too many questions for two papers
On the motion and acceleration of a free falling body
When exploring acceleration, objects with different masses are subject to different gravity, but the acceleration of objects moving freely is g. this conclusion seems to indicate that acceleration has nothing to do with the force and mass of objects?
Your idea is obviously wrong We say that the formula of G is g = f / m, where f is the universal gravitation of the earth to the object, and M is the mass of the object. With the increase of the mass m of the object, the size F of the universal gravitation of the earth sold by the object is also increasing. Coincidentally, in our scientific experiments, different arrays (and single measured quantity FN and Mn) are in a constant ratio relationship, and the ratio is the constant value G
But it should be noted that the earth is not a regular sphere, so objects in different places have different distances from the center of the earth, so they are subject to different gravitation. That is to say, the size of F is different, and the mass of the object does not change with the change of the place, so the value of G = f / m will change. Therefore, it is concluded that the acceleration of gravity on the earth will change with the change of the geographical position
Seek the North Teacher Edition seventh grade mathematics first volume one yuan one time equation exercise question 50!
Example: 3x = 8 + 2 (X-7) is such a question, 50 questions (must be one yuan!
7(2x-1)-3(4x-1)=4(3x+2)-1; (5y+1)+ (1-y)= (9y+1)+ (1-3y); 20%+(1-20%)(320-x)=320×40% 2(x-2)+2=x+1 2(x-2)-3(4x-1)=9(1-x) x/3 -5 = (5-x)/2 2(x+1) /3=5(x+1) /6 -1 (1/5)x +1 =(2x+1)/4 (5-2)/2 - (4+x)/3 =...
On the acceleration of free falling body
There is a question like this: two iron balls of equal volume fall freely from the top of the same tower at the same time, one of which is solid and the other is hollow?
Analytically, the air resistance is the same because of the same volume, while the solid iron ball is subject to larger external force, so the acceleration is larger than that of the hollow iron ball, so the solid iron ball lands first
If we analyze according to the greater resultant force and acceleration, we will come to the wrong conclusion that the greater the gravity is, the greater the acceleration is when there is no air resistance
So what should be the correct analysis of this problem? How not to violate the existing conclusions?
According to what you said, when two objects have equal mass and air resistance, the greater the resultant force is, the greater the acceleration is. What's the difference between the resultant force and the mass? It's a solid and a hollow
Czj540 what you said is reasonable, but I still don't understand why it is related to gravity in this case?
There are some problems in the analysis. The deceleration is not only related to the external force, but also related to the mass of the object. There is no doubt about F = ma (Newton's second law)
It is wrong to say that the greater the external force, the greater the acceleration
Let's go back to this problem. For any free falling object:
G-f=ma
So: a = (G-F) / M = G-F / M
The resistance of the two balls in the process of falling is equal, and the mass of the solid iron ball is larger, so the acceleration of the solid iron ball in the process of falling is larger. According to h = 1 / 2at ^ 2, the solid iron ball falls first. (in addition, it should be pointed out that the effect of air resistance is considered in the process of free falling, so the process of falling is not free falling motion.)
Beijing Normal University Press grade 7 mathematics volume 1 96 pages
Junior high school mathematics teacher's book grade 7 Volume 1 Chapter 2 rational numbers and their operations review questions
The answers are as follows:
What is free falling acceleration?
Acceleration of gravity [definition] the acceleration of an object near the earth's surface under the action of gravity is called acceleration of gravity, also called acceleration of free falling body, expressed by g. the direction of acceleration of gravity g is always vertical and downward
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