Use the four numbers of 1, 4, 7 and 8, and use +, -, *, / or brackets to form the formula of 24

Use the four numbers of 1, 4, 7 and 8, and use +, -, *, / or brackets to form the formula of 24

(7÷1-4)*8=24

Let f (x) be the density function of a continuous random variable, a > 0. Prove: ∫ [f (x + a) - f (x)] DX = a from the product of negative infinity to positive infinity!
This is the knowledge of random variables and their distribution in the second chapter of probability theory

F is the density function and F on the right is the distribution function
It is proved that the property of continuity is not needed
∫[F(x+a)-F(x)]dx=∫∫_ {x

One tenth of 9 meters is equal to what part of 1 meter?

Nine tenths

How to solve the equation of X △ 2-x △ 5 = 90?
OK, I'll give you a five point reward

X÷2 － X÷5 ＝ 90
Multiply each by 10
5X-2X=900
3X=900
X=300

sin²1°+sin²2°+sin²3°+...+sin²88°+sin²89°+sin²90°=
How much is it?

Sin21 ° + sin22 ° + sin23 ° +... + sin288 ° + sin289 ° + sin290 ° sin21 ° + sin22 ° + sin23 ° +.. + sin244 ° + sin245 ° + cos244 ° + cos243 ° +... + cos21 ° + sin290 °. According to the formula sin2a + cos2a = 1, it is obtained that = 1 + 1 +.. + 1 / 2 + 1 = 45.5

There are 56 pages in a book, 3 / 8 of which are () pages

21 pages

What is Tan (- 4 / 7 π) equal to

1
Because the tangent is a function with pie as period. Add 2 pie to it and get Tan (- 7pi / 4) = Tan (PI / 4)
Just look up the trigonometric function table and get 1

Given p (4, - 9) Q (- 2,3), find the ratio of the intersection of Y-axis and line PQ to the vector PQ

The equation of line PQ is as follows:
y-3=[(-9-3)/(4+2)](x+2)
That is: y = - 2x-1
Let x = 0, then y = - 1
The intersection coordinates of PQ and Y axis are (0, - 1)
P (4, - 9) Q (- 2,3)
Let the intersection of the line PQ and the y-axis be divided into the ratio λ of the directed line PQ
From the fixed score point formula: 0 = [4 + (- 2) λ] / (1 + λ)
The solution is: λ = 2
The ratio of PQ to PQ is 2

Prove the periodicity of function
y=f(x)
1. For x = a, x = B symmetry, t = 2 Ⅰ A-B Ⅰ is proved
2. For (a, 0) (B, 0) symmetry, it is proved that t = 2 Ⅰ A-B Ⅰ
It is proved that t = 4A is symmetric and odd for x = a
4. For x = a symmetry and even function, t = 2A is proved
Just prove the first one and the second one. We need words, not images

Find the equation of straight line passing through the intersection of x-2y + 1 = 0 and 2x + 3Y + 9 = 0 with equal intercept on the coordinate axis

From X − 2Y + 1 = 02x + 3Y + 9 = 0, the intersection coordinates of x = − 3Y = − 1 ℅ line x-2y + 1 = 0 and 2x + 3Y + 9 = 0 are (- 3, - 1). ① when the line passes through the origin, the equation satisfies the condition that y = KX, then - 3K = - 1, k = 13, and the linear equation is y = 13X. ② when the intercept of the line on the coordinate axis is not 0, the equation is XA + Ya = 1, (a ≠ 0), then − 3A + − 1A = 1, and the solution is a=- 4, then the linear equation is x + y + 4 = 0. To sum up, the linear equation obtained is y = 13X or x + y + 4 = 0