Why is this question like this, and why is the answer to this question this?

x²+3xy-2xy-6y² -2x²+8xy+2xy-8y²-x²+11xy-14y²

Among them, the calculation of -2 (x-y) (x-4y) should be multiplied by -2, because it also needs to be calculated with (x-2y) (x+3y), so it is called "simplification first"! The result of simplification is: -x +11xy-14y, and then substituting the value of x y into 33 when evaluating the value.

From a personal point of view, I haven't touched a math problem for more than 10 years, I hope it can help you! )

This is a polynomial multiplied by a polynomial, multiply a part first, the original = (-2x+2y)(x-4y)-2x +8xy+2xy-8y

2x²+10xy-8y²

2x²-10xy+8y²

-2[(x-y)(x-4y)]=-2[x²-xy-4xy+4y²]=-2[1+10+16]=-54

Count the brackets first, multiply each term separately, the same sign is positive and the difference sign is negative, and finally substitute the value, and then multiply by -2

2(x-y)(x-4y)=-2(x2-5xy+4y2)=-2x2+10xy-8y2, and then substitute x=-1, y=2, what's the problem.

The snake is gone, so the mongoose will be gone, this is true, because the mongoose has nothing to eat, so it starves to death, so A is a mongoose.

The main judgment is rats and plants, because rats have no natural predators, so the number of rats will increase at the beginning, you observe the timeline from 0, which means that from the moment the snake disappears, it should be more rats, fewer plants, fewer plants, and fewer mice.

Yes, you can think of it this way, divide each piece of mooncake into five parts, the first piece you eat one part, and the second piece you also eat one, so that you eat 2 portions, for one piece of mooncake, you eat 2/5According to the meaning of the fraction, divide a piece of mooncake into 5 equal parts, and you take the equivalent of 2 of them, so it is 2 5

Judgment: Zhao Qian, Sun Li.

Original words: It's not me, it's Lee, it's money, I didn't donate.

According to a analysis: Truth False Tell Donate (False) Truth.

Result: 2 true and 2 false, not meeting the condition of 1 true.

According to b analysis: donate (false) falsely falsely true.

Result: 1 true and 3 false, and 1 true condition was met.

According to C analysis: Truth Truth Truth False Donation (Falsehood).

Result: 2 true and 2 false, 1 true condition is not met.

According to D analysis: Truth Truth Truth False Donation (Falsehood).

Result: 2 true and 2 false, 1 true condition is not met.

Therefore B is correct, Lee is telling the truth, and everything else is false.

Zhao said: "I didn't donate it".

The money said, "It was donated by Li."

Sun said, "It was donated by money."

Lee said, "I definitely didn't donate. ”

Assuming that Zhao is telling the truth, Qian and Li speak contradictory.

Assuming that Qian is the truth, Qian and Li speak contradictory.

Assuming that Sun is telling the truth, Qian and Li speak contradictory.

Assuming that Li is telling the truth, it can be speculated that it was donated by Zhao. This question is selected B

Because: C Qian is telling the truth, and it was donated by Lee.

d What Lee said was a lie, and it was donated by Lee.

One of them must be true, so what A and B say is false.

x (60 + 70) is the time when B and C meet, and x (50 + 70) is the time when A and C meet, because C meets B for 2 minutes and then meets A, so the time for B and C to meet plus 2 points is equal to the time when A and C meet.

A walks 50 meters per minute, B walks 60 meters per minute, C walks 70 meters per minute, A, B, A, and C start in the same direction at the same time, C meets B, and then meets A. a, b ().

Solution: You can do that as well.

Suppose it takes x minutes for C and B to meet.

Equal according to the whole process.

60+70）x=（50+70）（x+2）130x=120x+240

10x=240

x=24, so the whole course = 24 (60+70)=3120 meters, I am the teacher, thank you for adopting.

It is based on the difference of 2 minutes in the encounter time, and find the equation of the equivalence off.

Of course, you can also directly set the mass fraction of sulfuric acid to x%, then the mass of sulfuric acid is 100x g, that is:

h2so4+zn=znso4+h2↑

100x100x/98=x=

Find a cube, cut it diagonally, and the obtained ZHI section is cdc1d1 in the figure, which is not a square version of dao, it is a rectangular weight, cd=c1d1, they are actually diagonal lines on a certain surface in the original cube, in this way, they and the two right-angled sides form an isosceles right-angled triangle, and the side of the triangle is the edge length of the square, so that the length of cd is 2 times that of the right-angled side.

Draw a shaft section and do it proportionally using the corresponding sides of similar triangles.