Probability of independent events A. B is two random events, P (a) = 0.5, P (b) = 0.3, if a and B are independent Then p (AUB) =?

Probability of independent events A. B is two random events, P (a) = 0.5, P (b) = 0.3, if a and B are independent Then p (AUB) =?

p(AUB)=P（A）+P（B）-P（AB）
=0.5+0.3-0.5*0.3
=0.8-0.15
=0.65

In a class, four of the five boys and three girls should be selected to participate in a certain community service. If at least one girl is required, then the probability of two girls out of the four selected is 0___ ．

According to the meaning of the question, there are C84 ways to choose from any four of the eight people, but there are no girls, that is, all boys have C54 ways, then at least one girl has c84-c54 = 65 ways; there are exactly two girls, that is, two boys and two girls have C52 × C32 = 30 ways; then the probability that there are exactly two girls in the four people is 3065 = 613; so the answer is 613

Why is an event with a probability of 0 not necessarily impossible?

Probability theory says that the probability of an impossible event is zero, but a zero probability event can happen. For example, if you take a person in the universe, you can get your probability. This is an example of a zero probability event that can happen!
Random variables are divided into continuous and discrete, and their distribution descriptions are different
For continuous random variables, the probability density value of a specific point is a bounded constant, which can be arbitrary (including 0 and 1). But because the point has no length, the probability density integral of the point is 0 (because the probability density value of the point is bounded), that is, the probability of the event corresponding to the point is 0, but the event is still possible, Similarly, the probability density of a point is 1, but the probability density integral of the point is still 0, so the event with probability 1 does not necessarily occur. In short, for continuous random variables, it is meaningless to discuss the probability of a single point (all are 0), The probability that the random variable falls into an interval
For a discrete random variable, if its event domain is finite, it can be considered that the event with a probability of 0 will not happen, and the event with a probability of 1 will happen. But if the event is infinite, it needs to be analyzed concretely
Since all events with zero probability are possible, events with zero probability are possible. However, when we deal with problems, events with zero probability are regarded as zero probability events, which are not absolute

The velocity formula of uniform variable speed linear motion is

V = VO (initial velocity) + at
Understanding and application: ① a t is the change of velocity over a period of time △ V: vector
Analysis: when a and V0 are in the same direction, no matter how a changes, the object still accelerates
When the directions of a and V0 are opposite, no matter how a changes, the object still decelerates
I analyzed it myself

A mixed operation problem of rational number is designed by ourselves. The problem should meet the following conditions at the same time:
It must contain five operations: addition, subtraction, multiplication, division and power
Divisor must be a fraction
The base of a power operation must be a negative fraction
The result is equal to 2013

((-1/2)^3)/(1/8)+3000-493*2

How to deduce the formula of instantaneous velocity at the midpoint of the displacement of uniform linear motion

Let the total distance of initial velocity V0 and final velocity VT be s acceleration AVT = V0 + at S = v0t + (1 / 2) at ^ 2. Substitute the former formula into the latter formula and eliminate t to get: 2As = VT ^ 2-v0 ^ 2. If we find the midpoint velocity V, then 2A * (s / 2) = as = V, then ^ 2-v0 ^ 2 will eliminate as and get VT ^ 2-v0 ^ 2 = 2

Please write a mixed operation problem of rational number and solve it
1. It must include five operations: addition, subtraction, multiplication, division and power
2. Divisor must be a fraction
3. The base must be a negative fraction in the power operation
4. The calculation result is equal to 2013

-2013 times (- 1) + 1 / 4 divided by the square of (- 1 / 2) - 1

Derivation of instantaneous velocity at intermediate position
RT,

Middle position velocity V1 final velocity V2 initial velocity V3
V1*V1-V3*V3=ax
v2*v2-v3*v3=2ax
v1*v1=ax+v3*v3=(v2*v2)/2+v3*v3=(v2*v2+v3*v3)/2
Re prescribing

Compile a rational number mixed operation problem, and meet the following conditions
1. With addition, subtraction, multiplication and division power 2, divisor is negative fraction 3, power base is with fraction 4, the result is 2009

14:278 363 * (8:4) ^ 2 + (- 502:2) / (- 1:2) - 1 = 2009
I hope it won't be wrong. I'm the one to blame for the wrong watch

Time and time, displacement and position
Is time time? No. since time is not time, then time means time is 0. How does time constitute time? Is there distance between points? If there is distance between points, then location is also distance? That is displacement? If not, then how can there be the sentence "points constitute a line"? Here is instantaneous velocity and velocity Then I don't understand the definition (definition: the speed of a particle at a certain time or position) there is a speed of an object at a certain position or time? (definition of speed: the speed is equal to the ratio of displacement and the time taken for displacement.) don't be too official

Definition of velocity: velocity is equal to the ratio of displacement to the time taken for displacement. This is the average velocity. Instantaneous velocity is calculated by the average velocity, v = x / T. instantaneous velocity is the velocity at a certain moment, that is to say, t is very small, almost o (limit thought, which will be learned later). Velocity is the abbreviation of instantaneous velocity