It s a bit difficult to explain a math problem in the first year of junior high school

The demeanor of the master is extraordinary.

In 2x 3-x 2-5x+k, there is a factor of (x-2), which means that 2x 3-x 2-5x+k is divisible by (x-2), and there are at least two ways to find it:

1. Factoring method:

2x^3-x^2-5x+k

2x 3-4x 2)+(3x 2-6x)+(x+k)2x 2(x-2)+3x(x-2)+(x+k) The first two groups have the factor x-2, so x+k can also be divisible by x-2, obviously k=-22, with remainder division: divided = divided*quotient + remainder, divisible remainder = 0, so substituting x=2 can obtain the above formula = 0, that is, the divided formula 2x 3-x 2-5x+k=2*2 3-2 2-5*2+k=0, the solution; k=-2

You can also find the cofix by dividing the polynomial by the polynomial, which can also be found by the equivalence = 0.

Divide (2x 3-x 2-5x+k) by (x-2) to get another factor (2x 2+3x+1).

k=-2

Because the original form has a factor, the original form must be split into.

2x(3)-4x(2) =2x(2)(x-2)3x(2)-6x =3x(x-2)

x +k =x-2

Therefore k = -2

1.Solution:

Direct sales: profit is: (120-80) x-3000 = 40x-3000 consignment sales: profit is: (105-80) x = 25x

2.Solution: 40x-3000=25x

then x=200 (pieces).

3.Solution: Let 40x-3000>25x, then x>200

So: when x > 200, direct sales profit is more.

When x=200, the direct sales profit is equal.

When x<200, the consignment profit is more.

The general formula for each item is: 1 n*(n+1).

I guess it's going to be here.

That is, the original formula = 1 1 * (1 + 1) + 1 2 * (2 + 1) + 1 3 * (3 + 1) + ....1/1998*（1998+1）

Count the rest yourself. You can't help it anymore.

I'm not sure, right.

If Mr. Wang receives a manuscript fee of 2,400 yuan, it is within the range of (2).

That is, taxable: (2400-800)*14%=224 (yuan) Answer: Mr. Wang should pay 224 yuan.

If you get 4,000 yuan, it is miscellaneous in the range of (3).

That is, taxable: 4000 * 11% = 440 (yuan) Answer: Mr. Wang should pay 440 yuan of tax.

and 420<440, and the manuscript fee is not higher than 800 yuan, no tax is paid;

Therefore, Mr. Wang's manuscript fee should be within the range of (2).

Mr. Wang's manuscript fee is X yuan.

x-800)*14%=420

x-800=3000

x=3800

A: Mr. Wang's manuscript fee is 3,800 yuan.

ps: Solve 8 knows, yes. Blame me for 8 wrongs. -

Hey, I've been studying for nothing for the past three years.

Shadow area = area of the large parallelogram - area of the small triangle above = 6 10-4 6 2 = 48, i.e. 48cm?

Solution: ABC def

abc=abhe ceh

def=cdhf＋ceh

abhe=cdhf

ab=de, and hd=4

he=ab-dh

4 The translation distance is 6

be=6 and abhe (eh+ab)be

abhe=½×（6+10）×6

48∵abhe=cdfh

cdfh=48

That is, there is an area of 48 square units.

The area of the parallelogram DHCF is equal to the area of the trapezoidal ABEH, HE is equal to DE-DH=6, the upper base plus the lower base multiplied by the height divided by two, i.e. (HE+AB) EB 2, (6+10) 6 2, answer 48, there is no unit.