2 (X-Y) (x + y) - (x + y) square,

2 (X-Y) (x + y) - (x + y) square,

In 2 (X-Y) (x + y) - (x + y) 2, (x + y) can be extracted as a whole. SO 2 (X-Y) (x + y) - (x + y) 2 = (x + y) {2 (X-Y) - (x + y)} = (x + y) (x-3y)

Square of (x's square + Y's Square) - square of X's Square ~ common factor of Y

(x^2+y^2)^2-x^2y^2
=(x^2+y^2)^2-(xy)^2
=(x^2+y^2+xy)(x^2+y^2-xy).

Decomposition of common factor (x + y) ＾ 2-4xy

（X＋Y）＾2－4XY
=x^2+2xy+y^2-4xy
=X^2-2XY＋Y^2
=(x-y)^2
I don't know. I'm also a sophomore in junior high school. I didn't learn it

The price of a computer increased by 3 / 10 and then decreased by 2 / 5. Now the price of each computer is 3000 yuan. How much is the original price of each computer?
Use a linear equation of one variable

3000 / (3 / 5) = 5000 yuan before lowering
If the original value is x yuan, there will be
X (1 + 3 / 10) = 5000 yuan
We can get x = 50000 / 13 = 3846 yuan

(- 4B's square + 4AB) / (4a-4b)

If the value of 2x + | 4-5x | + | 1-3x | + 4 is constant, find the condition that x should satisfy and the value of this constant

The condition that x should satisfy is: 4 − 5x ≥ 03x − 1 ≥ 0, the solution is 13 ≤ x ≤ 45, the original formula = 2x + (4-5x) + (3x-1) + 4 = 7

Ma Zhe's discrimination: productivity can develop by leaps and bounds,

That's right. For example, Tibet in China was serfdom before liberation, which was equivalent to feudal society. After liberation, it directly entered socialist society, and the productive forces increased greatly. This is the leap forward development of productive forces

The sum of the numbers in a three digit number is 11. If you transpose the number in a hundred with the number in a single digit, the number you get is 693 larger than the original number
The digit of the digit and the digit of the digit are transposed. All the digits are 54 larger than the original number. Find the three digits. (use the system of linear equations with three variables)

Let the hundreds be x, the tens be y, and the ones be Z
Then: x + y + Z = 11 (1)
100z + 10y + x -(100x + 10y +z) = 693
==> 99z - 99x = 693
==> z - x = 7
==> x = z-7 (2)
100x + 10z + y -(100x +10y + z) = 54
==> 9z - 9y = 54
==> z - y = 6
==> y = z-6 (3)
Substitute (2) and (3) into (1)
z-7 + z-6 + z = 11
==> 3z = 24
==> z = 8
Substituting the solution in (2), we get x = z-7 = 8-7 = 1
Substituting the solution in (3), y = Z-6 = 8-6 = 2
So this three digit number is 128

How to choose the stock on page 20
It's from pep
Stock trading is a kind of financial activity in market economy. It can promote investment and capital circulation. The highest price of a stock on Nanjing stock exchange is 0.2 yuan higher than the opening price on the first day. The lowest price is 0.3 yuan lower than the opening price. The highest price on the second day is 0.3 yuan higher than the non opening price. The lowest price is 0.1 yuan lower than the opening price The highest price of the third day is equal to the opening price. The lowest price is 0.2 yuan compared with the opening price. The difference between the highest price and the lowest price in a day is called the increase calculation. This is the average increase of the day, with two decimal places reserved

Higher than the opening price is a positive number, lower than the opening price is a negative number. The first day's increase is 0.2 - (- 0.3) = 0.5, the second day's increase is 0.3 - (- 0.1) = 0.4, the third day's increase is 0 - (- 0.2) = 0.2 (0.5 + 0.4 + 0.2) / 3 = 0.37

As shown in the figure, P is any point on the diagonal BD of the parallelogram ABCD. The straight line passing through point P intersects ad at point m, BC at point n, the extension of BA at point E, and the extension of DC at point F

It is proved that: ∵ quadrilateral ABCD is a parallelogram, ∥ ab ∥ CD, ad ∥ BC, ∥ BPE ∥ DFP, ∥ PE: pf = Pb: PD, ∥ ad ∥ BC, ∥ BPN ∥ DPM, ∥ Pb: PD = PN: PM, ∥ PE: pf = PN: PM, that is PE · PM = PF · PN