Math problems, rushed to death, picked up dead.

If the age of the younger sister is x this year, then the age of the elder sister is x+8.

x-4=1/2(x+8-4)

x=12x+8=20

So this year's sister's age is 12 years old, and this year's sister's age is 20 years old.

Let the age of the sister be x, then the age of the sister is x 8, and the equation is x 4 (x 8 4) 2, and then solve it to get the age of the sister, and subtract 8 is the age of the sister.

Sister is x this year, and sister is y years old this year.

x-y=81/2(x-4)=y-4

Results x = 20 and y = 12

A: My sister is 20 years old and my sister is 12 years old.

Suppose the elder sister is x years old this year and the younger sister is y this year, and the solution is x-y=8, (x-4)=2(y-4), and the solution is x=20, y=12. Or use the unary equation to solve, let the sister be z years old this year, then the sister (z+8) is 2 (z-4) = z+8-4, and the solution is z = 12, so the sister is 12 years old this year, and the sister is 12 + 8 = 20 years old.

x-1 2x=8 Solution: (1-1 2) x=8 x=16 Sister this year: 16+4=20 (years old) Sister this year: 20-8=12 (years old).

Solution: Suppose: My sister is x years old this year, and my sister is (x+8) years old this year.

2（x-4）=(x+8)-4

Solution: x=12

So: (x+8)=12+8=20 years.

A: My sister is 12 years old and my sister is 20 years old.

Set the younger sister to be x years old.

x+8)-4=2(x-4)

The solution is x=12

So my sister is 20 this year, and my sister is 12 this year

When the quotient is constant, the dividend is proportional to the divisor.

When the product is constant, the two factors are inversely proportional.

When one factor is constant, the product is inversely proportional to the other.

When a is certain, b, c are proportional.

When b is certain, a, c are proportional.

When c is certain, a, b are inversely proportional.

When A is constant, B is proportional to C, and when C is constant, A and B are inversely proportional.

When b is constant, a and c are proportional.

Analysis: When c must be the product is certain, so a and b are inversely proportional, when a is certain, because c b = a is a must be the ratio so b is proportional to c.

If b is certain, it means the same as it.

When the quotient is always proportional to the divisor.

When c is certain, a and b are inversely proportional.

When b is constant, a and c are proportional.

When A is certain, B and C are proportional.

If abc is not 0 and a b = c, when b is constant, a is proportional to c; When C is constant, A and B are inversely proportional, and when A is definite, C and B are directly proportional.

c must be inversely proportional to ab.

A or B must be proportional to B and C or A and C.

The image of the function y=x-(m-2)x+m passes through (-1,15) and is substituted by (-1,15), 15=1+m-2+m, m=8 The image intersects with the x-axis a,b

That is, find the solution of x - (m-2)x+m=x -6x+8=0, x=2,4a(2,0), b(4,0)ab=2

S triangle abc = 1 2 ab height = 1 point c to x-axis distance = height = 1 point c ordinate is 1 or -1

The ordinate is 1, x -6x + 8 = 1, the solution is x = 3 2, the coordinate of point c (3- 2, 1), (3 + 2, 1).

The ordinate is -1, x -6x+8=-1, and the solution is x=3, and the coordinates of point c (3,-1)point c can be (3-2,1), (3+ 2,1), (3,-1).

Substituting (-1,15) into y=x -(m-2)x+m, 15=1-(m-2)*(1)+m, we get m=8

The original function is: y=x -6x+8 so that y=0 gets the intersection of the image with the x-axis: (2,0)(4,0).

The distance of the AB point = 4-2=2, so the height of the triangle ABC corresponding to the base of AB = 1 (1 2*2)=1

This height is also the absolute value of the ordinate of point C.

i.e.: x 2-6x+8=1 or x 2-6x+8=-1 from x 2-6x+8=1 gets: x=3+ 2 or x=3- 2 gets from x 2-6x+8=-1:

x=3 so the coordinates of c (3 + 2,1).3- 2,1) or (3,-1).

Substituting (-1,15) first to solve m=8, we can get y=x 2-6x+8=(x-2)(x-4).

Therefore, the coordinates of the two points of ab are (2,0)(4,0).

ab=2 and area 1

then the height is 1, i.e. the absolute value of y is equal to 1

When y=1 is y=x2-6x+8

The solution is x1 = 3 + root 2, x2 = 3 - root 2, and the coordinates are (3 + root 2,1) and (3-root 2,1).

When y=1 is y=x2-6x+8

The solution is x=3, so the coordinates are (3,-1).

Substituting the point (-1,15) into the function expression gives m=8, so that the function expression y=x 2-6x+8 is obtained, and the image has two intersection points with the x-axis, that is, the point of y=0, and we get a(4,0),b(2,0).

The area of the triangle ABC is equal to the distance between the two points of AB multiplied by the ordinate of point C, so the ordinate of point C is equal to 1 2, and because point C is on the function image, the coordinates of point C satisfy the function relationship, that is, 1 2=X 2-6X+8 so that the X coordinates of point C are obtained. The result is (three plus minus half of the root number three,0). There is also a point on the axis of symmetry x=3,y=1

Since the image of the function y=x-(m-2)x+m passes through (-1,15), there is 15=1+m-2+m to get m=8

i.e., y=x -6x+8, so that x -6x+8=0 gives x=2 or x=4, i.e., the image intersects with x-axis at (2,0), (4,0). The ab spacing is 2, and because the s triangle abc 1=1 2*ab*|y|, gotta |y|=1, y=1 or y=-1

When y=1, there is x -6x+8=1 to get x=3 + root number 2, or x=3 - root number 2 when y = -1, there is x -6x + 8=-1 to get x=3 so, the coordinates of point c are (3, -1), (3 + root number 2, 1), (3 - root number 2, 1).

First bring (-1,15) into m-2)x+m to get y=x -6x+8, get a(0,2), b(0,4) and then ab=2, s=2 to get higher than 1, bring y=+1 into the equation, and get three numbers, which are the coordinates of point c.

From the image through (-1,15), substitute the m=8 of the equation so the functional equation is y=x 6x+8 so the coordinates of point a b are (2,0) and (4,0) let c(x,y) get 2| from s=1y|2=1 gives y=1 or -1

Substituting the function equation respectively to get x=3 or x=3 + root number 2 or 3-follow 2

1. Solution: Assuming that there are x girls in existence, then:

x-21）×9/4=5x/3-21

Solution: x=45

2. Solution: Assuming that the total length is x kilometers, then:

2(x-54)=80% x

Solution: x=90

3. Solution: (1) If it takes x minutes for two people to meet for the first time, then:

200+150) x=400

Solution: x=8 7

2) If it takes Y minutes for two people to meet for the first time, then:

200-150) y=400

Solution: y=8

1.Let the number of boys be x, and the original total number of students will be y

x/y=5/9

x/(y+21)=2/5

Solution x=30

y=54 for existing girls: 54-30+21=45

2.A walks the whole course and B walks 80%.

A walks halfway and B walks 40%.

So x = 90 km.

3.B is 1-1 4 times faster than A

I'll count it as 3 or 4 times.

Set off in the opposite direction. 400 (200+ minutes.)

Access to departure (t = 8 minutes.

Ask the question, and that's it.

Are you sure you're right?

1.Set up the existing female x people, from the known.

x-21)/(1-5/9)+21=x/(1-2/5)(9x-189)/4+21=5x/3

27x-189*3+21*12=20x

7x=189*3-21*12

Solve the equation to get x=45

2.When car A is halfway through, car B is still 54 kilometers away from point B, and when car A reaches place B, car B has traveled 80% of the way

Therefore, when car A travels half of the journey, car B travels 40% ab of the whole journey, and the distance between the two places is 54 60% = 90 (kilometers).

3.B's speed is 200*5 4=250 (m/min) (1)400 (250+200) approx. = min) (2)400 (250-200)=8 (min).

1. If there are 3x female students now, then there will be 3x-21 female students and 2x male students in the last semester, and the column equation 2x: (3x-21+2x)=5:9

Solution, x=15

2. According to the title, when car A is halfway through, car B is 40% away, that is, the whole journey is -54, let ab=s(s-54): s=40% s=90

3, How many times faster is B than A? I didn't understand it.

The first question is set to have a total of x people, which is now x+21 people, and the number of boys remains the same, so 5 9x=2 5(x+21), the original total number of x is 54, and the number of girls is 24, and now the number of girls is 24+21=45.

Question 2: 54 (40%)=135

Question 3: 8 minutes.

1.Let the boy x x ( x=30 girl 452, v2= x 2v1=(x-54) v2 solve x=9032(1) v B = 150 meters per minute in the opposite direction t = 400 (200 + 150) = 8 7 minutes.

2) In the same direction: t=400 (200-150)=8 minutes.

Question 1: There are 45 female students, and there are 75 in total.

Question 2: The total length is 90 kilometers.

3 questions: 7 minutes.

Minute.

Solution: Suppose the annual interest rate of the depositor's stored seeds is x.

The money withdrawn from the interest principal.

Interest Principal Annual Interest Rate Number of years.

According to the above formula, the equation can be listed:

12000x＋12000＝12240

12000x＝240

x＝x＝2％

A: The annual interest rate of the depositor's deposit is 2.

I don't know, I'm still a little baby.

Speechless....

2.Divide by minus a quarter (minus four and a half of the grandson of the search chain) multiply by two-ninth.

Stupid rent - 9 2) * (2 9) (1 4).

Set younger brother x and sister y years old.

then x+y = 40

y-x=3 gives x= y=

y-x=3 because the older sister is always 3 years older than the younger brother.

Because my brother is 8 years old and my sister is 11 years old this year, the difference between my sister and brother is 3 years, and this will never change.

If the younger brother and sister are 40 years old, and the younger brother is x years old, the sister is x + 3 years old.

So sibling age and x+x+3=40

Get x = years, i.e. 18 and a half years old.

The elder sister is 21 and a half years old.

A: My sister is 21 and a half years old, and my brother is 18 and a half years old.

1. The number that can be composed and divisible by 3 is 1 3 of all numbers, so it can be composed of 9*8*7 3=168 such numbers.

2. If the product is a multiple of 6, then there is the following choice, x represents any number 2, 3, x

4，3，x6，x，x

8，3，x2，9，x

4，9，x8，9，x

So, the product of the chosen number is a multiple of 6, which is (1 9 + 7*(9*8)) = 15 72 for all combinations

So there are 9*8*7 *(15 72) = 105 permutations.

Because this selection has nothing to do with order, it is necessary to remove different combinations in the same arrangement. i.e. 105 3 = 35

Divide these 9 digits into three groups: 1, 4, 7, 2, 5, 8, 3, 6, and 9

1) How many 3 digits divisible by 3 can be composed Take a(3,3)=6 from 1,4,7

Both take a(3,3) =6 from 2,5,8 and both take a(3,3)=6 from 3,6,9

Or take one of the three groups: 3*3*3*a(3,3)=27*6=142 can form 180 3 digits divisible by 3.

2) Make the product a multiple of 6, and the last digit is an even number on the basis of (1).

2 out of 1,4,7.

There are 4 out of 2,5,8.

There are 2 out of 3,6,9.

Or one of the three groups: 3*3+3*3*2+3*3=36

The product is a multiple of 6, and there are 44 options.