Math problem help for junior high school, math study problem for junior high school

Equality relationship: number of boys * 3 = number of girls * 7

Boys = 170 - Girls.

Set: If there are x boys, then there are (170-x) girls.

3x=7(170-x)

3x=1190-7x

x=119170-119=51 (person).

A: There are 119 men and 51 girls.

This is relatively simple, so that each tree pit can be planted means that the number of trees dug by the boys and the number of trees planted by the girls is equal, and the number of unknown boys is x and the number of girls is y

then 3x=7y

x+y=170

The solution is x=119

y=51

Boys x, girls 170-x

The tree should be the same as the pit tree, so:

3x=7(170-x)

10x=7*170

x = 119 boys, 51 girls.

If there are x male students, then there are 170-x female students; The number of pits and trees are equal to the inscription.

3x=（170-x）*7

If the number of boys in the class is x, then the number of girls is 170-x

According to the meaning of the title, 3x=7*(170-x) gives x=119

So it's 119 boys and 51 girls.

Solution: There are x boys and 170-x girls.

3x=7(170-x)

The solution is x=119

170-x=170-119=51

There are x boys and 170-x girls.

3x=7(170-x)

3x=1190-7x

3x+7x=1190

10x=1190

x = 119 boys = 119 people.

Girls = 170-19 = 51.

5.(1) a/(1-60%)=

2) (x+y)/10

6.B: (h+20) m C: (h-30) m difference: 50 m.

7.The area of the rectangle: 2x*4=8x (square centimeters) The area of the trapezoid: (x+3x)*5 2=10x (square centimeters) 10x-8x=2x (square centimeters).

8.The materials used in both schemes are the same.

9.Second row: A+1

Third row: A+2

m=a+(n-1)

When a=20, n=19, m=20+18=38 (pcs)10Original: 10a+b, after exchange: 10b+a

SUM: 11a+11b=11(a+b) divisible by 11.

11.（1）48-a-(2a+3)-(a+2a+3)=48-a-3-3a-3

45-7a2) can not get a quadrilateral.

When a=3, the fourth edge = 24 cm. than before.

The sum of the three sides is larger, when a = 7, the fourth side = -4 cm, and the side appears negative, which is also not true.

12.（1）5（a+b)

2)11(x+y)^2-(x+y)

13.Let the second interior angle be x degrees.

3x+x+(x+15)=180

5x+15=180

5x=165

x=333x=99 x+15=33+15=48.

A: The first angle is 99 degrees, the second angle is 33 degrees, and the third angle is 48 degrees.

The first three are correct. Sections 4 and 5 are incorrect, because number 4 should be positive integers, and 0 and negative integers are collectively called integers. Finally, 0 is not the smallest rational number, because there are also negative rational numbers, such as -1, -3, and so on.

If |m-1|=m-1, then m( 1. Because both a positive number and an absolute value of 0 are equal to itself. So, m-1 0, i.e. m 1.

There are four of the 1234 pairs.

m-1 is greater than or equal to 0 m, and m is greater than or equal to 1

The third question is the wrong drop, the forgotten Mr. Zero is going to cry.

1: 3 (the 4th has 0, and the 5th has a negative number).

2: greater than or equal to.

3: False, and 0

There are three correct, which are the 1st, 2nd, 3rd. Zero is also an integer, but it is neither odd nor even. Greater than or equal to. Wrong, 0 is not satisfied.

3x-2y=m-1——（1）

x+2y=m ——2）

1) + (2) get.

4x=2m-1

x=1 2*m-1 4 – 1 2*m-1 4 -1, so m -3 2

1)-(2) 3.

8y=-2m-1

y=1 4*m+1 8 – 1 4*m+1 8 -1, so m -7 2

So m -3 2 satisfies the conditions.

Let's assume that the oil price of the previous month was x and the import volume was y

Oil price of this month = import volume of this month =

Growth rate = increase cost * 100%.

So this month's oil** growth rate relative to the previous month is.

Solution: If the oil import volume of the previous month is a, and the cost of importing oil in the previous month is b, then the oil import volume of this month is a(1-5%)=, and the cost of importing oil this month is b(1+14%)=, so the growth rate of this month's oil ** relative to the previous month =

Equation solution: The total distance traveled by this person is 125m = 125 1000; 3 s = 3/3600 h

Let the other train have a speed of x;

The equation is. 125/1000/（70+x) =3/3600x = 150 - 70

x = 80km/s

Let me explain to you, the question is not that the two cars met in 3 seconds, but that the person saw the train pass in 3 seconds. So the passenger's captain is not needed.

Let the speed of the train be x

The total distance should be the sum of the lengths of the two trains: 100 + 125 = 225 m, 70 km h = 175 9 (m s).

175 9 +x)*3=225 solution x=500 9(m s)=200km h

125/3 m/s=625/6 km/h

625/6-70=205/6 km/h= km/h

There is no need to count the conductor of the train he was on, it was used to confuse.

This question can be explained in this way: Let the speed of the train be xkm h. Assuming that the passenger does not move, the relative speed of the oncoming vehicle is (x+70).

x+70）=125m/3s=

So x=80km h

You can use relative speed, which is relatively simple.

From the nose of the machine to meet the tail of the machine completely through, the distance is 100 + 125 = 225m, the time is 3s, so the relative speed is 225 3 = 75m s = 270km h Since it is in the opposite direction, this speed is the sum of the speeds of the two trains, so the speed of this train is:

270-70=200km/h