（1）12x²-3y² （2）49（a-b）²-16（a+b）

（1）12x²-3y² （2）49（a-b）²-16（a+b）

12x^2-3y^2
=3(4x^2-y^2)
=3(2x+y)(2x-y)
49(a-b)^2-16(a+b)^2
=(7a-7b+4a+4b)(7a-7b-4a-4b)
=(11a-3b)(3a-11b)

Decomposition factor: 16 (a + b) & # 178; - 9 (a-b) & # 178;

16（a+b)²-9(a-b)²
=[4(a+b)+3(a-b)][4(a+b)-3(a-b)]
=(4a+4b+3a-3b)(4a+4b-3a+3b)
=(7a+b)(a+7b)

On a smooth fixed inclined plane with an inclination of 30 degrees, use two light ropes to cross the fixed pulley and connect them to the trolley. Objects a and B with mass of 2M and m are suspended at both ends. When the trolley is stationary, the two ropes are parallel and perpendicular to the inclined plane, as shown in the figure. Regardless of the friction of the pulley, the positions of a and B are changed. When the trolley is stationary and balanced again, the following answer is correct ()
A. The mass of the car is 5MB. The resultant force of the pulling force of the two ropes remains unchanged. C. the rope with the original vertical inclined plane is now in the vertical direction. D. the supporting force of the inclined plane to the car is reduced to half of the original

A. When the position is not exchanged, the tension of the rope pulling B is TB = mg, and the tension of the rope pulling a is ta = 2mg. For the trolley, the equilibrium condition is: m trolley gsin30 ° = TA = 2mg, then M trolley = 4m, and the supporting force of inclined plane trolley is n = m trolley gcos30 ° - TB = (23-1) mg. Therefore, a is wrong. B and C make a and B position exchanged

(1) Solution to a series of equations: two thirds of a number is 10 greater than one half of it. Find the number
(2) In a right triangle, one acute angle is 5 degrees less than 4 times of the other. What are the two acute angles
PS: within 15 minutes,
Title bar is also a question, see the question answer

1.(2/3)X-(1/2)X=10 X=60
2.X+(4X-5)=90 X=19 71

Why does the friction force have nothing to do with the speed of an object? Don't say it's an experimental result. Give me an abstract and understandable explanation

Friction is the force that hinders the relative movement of an object. You can imagine the contact surface as a gear. When you pull the object with force, because it is uneven, you have to use force. When the object slides, no matter whether you pull fast or slowly, the concave and convex degree does not change, and the force will not change. The book does not necessarily have the answer to the question you want. This is the result of the experiment, Do more questions, you will understand. But my visualization should help you understand

Solution of one variable linear equation 10 × 0.8-2 = x + X × 20%

10×0.8-2=x+x×20%
8-2=x+20%x
6=1.2x
x=5

When a seventh grade student was doing his homework, he accidentally knocked over the ink bottle, so that only the following words could be seen in an applied question: "the distance between a and B is 40km, the speed of motorcycle is 45km / h, and the speed of truck is 35km / h." Please complete this assignment and solve the equation

Let's meet in X hours. According to the meaning of the question: 45x + 35x = 40, the solution is x = 12

4.1 ^ 2 / 5, 3.8 ^ - 2 / 5, (- 1.9) ^ - 2 / 5 Comparison size

4.1^2/5
3.8^(-2/5)=(1/3.8) ^(2/5)
(-1.9)^-2/5=1.9^(2/5)
Obviously 4.1 > 1.9 > 1 / 3.8
therefore
4.1^2/5 > (-1.9)^-2/5 >3.8^(-2/5)

1+2-3-4+5+6-7-8+9+10.+997+998-999-1000,
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The original formula is 1 - (3-2) + (5-4) - (7-6) + (9-8) - (11-10) + -（999-998）-1000
=1-1+1-1+1-1+…… -1-1000
=0-1000
=-1000

There are two piles of sand in a construction site. The first pile is 85 tons more than the second pile. After 30 tons of sand is used in each pile, the first pile is twice as much as the second pile. There are two piles of sand in each pile

If the second pile of sand is x tons, the first pile is 85 + X
85+x-30=2*（x-30）
55+x=2x-60
2x-60-55-x=0
x=115
115+85=200