1. Given that ABCD ≠ 0, try to explain that at least one of AC, - ad, BC and BD is positive, and at least one of them is negative 2.(1=1/2+1/3+...+1/1998)(1/2+1/3+...+1/1999)-(1+1/2+1/3+1/1999)(1/2+1/3+...+1/1998) If the score is not enough, say it! But don't fool me with the wrong answer! I can see it! Do you know it?

1. Given that ABCD ≠ 0, try to explain that at least one of AC, - ad, BC and BD is positive, and at least one of them is negative 2.(1=1/2+1/3+...+1/1998)(1/2+1/3+...+1/1999)-(1+1/2+1/3+1/1999)(1/2+1/3+...+1/1998) If the score is not enough, say it! But don't fool me with the wrong answer! I can see it! Do you know it?

1. Suppose that all of them have the same sign, then - ad = negative number, the conclusion holds. Suppose that there is one or two different signs, the conclusion also holds, so the conclusion holds. 2 +1/1998＝a1/2+1/3+…… +1 / 1998 = B, so the original formula = a * (B + 1 / 1999) - (a + 1 / 1999) * b = AB + A / 1999-ab-b / 199

Data 1: according to the statistics of authoritative departments, the per capita cash income of farmers in the first three quarters of 2011 was 2100 yuan. It is estimated that the income in the fourth quarter was the same as that in the first three quarters, and the annual income growth rate was about 12% compared with that in 2010. Data 2: according to the person in charge of the Ministry of agriculture of the people's Republic of China, the main factors of increasing farmers' income in 2011 came from three aspects: first, the national policy of benefiting farmers made the per capita income of farmers increase The second is that the price of agricultural products generally rises, which makes the per capita direct income of farmers increase by 200 yuan; the third is that the expansion of planting area brings about the increase of output? (2) According to the person in charge of the Ministry of agriculture, the goal of per capita income of farmers in 2012 is to increase by more than x% on the basis of 2011, but the policy factor will reduce the income by one percentage point compared with 2011, and the price will not increase any more. The growth rate of the third factor is the same as that of 2011. Therefore, we must rely on further implementation of industrial structure adjustment and other measures to increase farmers' income, If these other measures can increase the income of Y yuan, please determine the functional relationship between Y and X, and find out when y is not less than 70 yuan, what is the lowest per capita growth rate of farmers?

(1) The income of the fourth quarter is 2100 △ 3 = 700 yuan, and the income of the whole year in 2011 is 2100 + 700 = 2800 yuan. Suppose the income of 2010 is a yuan. According to the title, a (1 + 12%) = 2800, the solution is a = 2500. Therefore, due to policy factors, the per capita income of farmers is 2500 × 2% = 50 yuan

1: X / 2 = x / 3 + 22: 7-5x / 3 = 5-2x / 23: the solution of the equation (2 + x) / 3 = (x + 2) / 5 is the solution of the equation (AX + 3) / 3 = 5 + 2aX. Finding a 4: given that the complementary angle of an angle is four times of the residual angle, finding the degree of the angle

1, X / 2-x / 3 = 2x / 6 = 2x = 122, 7-5 = 5x / 3-2x / 22 = 2x / 3x = 33, solve the equation (2 + x) / 3 = (x + 2) / 55 (2 + x) = 2 (x + 2) 3x = - 6x = - 2, substitute x = - 2 into (AX + 3) / 3 = 5 + 2aX, get: (- 2A + 3) / 2 = 5-4a-2a + 3 = 10-8a6a = 7a = 7 / 64, let this angle be x, then its complementary angle and residual angle are respectively

It is known that y = (X & # 178; + 6x + 9 / X & # 178; - 9) / (x + 3 / X & # 178; - 3x) - x + 3. Try to explain that no matter x is any meaningful value, the value of Y will not change
The portrait project can be completed in 12 days with the cooperation of company a and company B, and the total construction cost is 102000 yuan; if the project is completed by company a and company B alone, the time taken by company B is 1.5 times that of company a, and the daily construction cost of company B is 1500 yuan less than that of company A
(1) How many days will it take for the two companies to complete the project independently
(2) If a company is allowed to complete the project alone, which company will have less construction cost [OK, everyone will know this problem]
If (1 / M) + (n / 1) = (7 / M + n), what is the value of (n / M) + (M / N),
If a and B are real numbers, and there is always a / (X-5) + B / (x + 2) = (5x-4) / (X & # 178; - 3x-10), then a=____ ,B=____ [same as filling in the blank,
Can the value of fraction (x + 3) / (X & # 178; - X-12) be equal to 0? Please explain the reason
PS: answer format:
    ……
……
……
And so on, I believe you will understand

1. Y = [(x + 3) &# (178; / (x + 3) (x-3)] × [x (x-3) / (x + 3)] - x + 3 = [(x + 3) / (x-3)] × [x (x-3) / (x + 3)] - x + 3 = x-x + 3 = 3 〈 no matter what the value of X is, the value of Y remains unchanged. 2. (1) suppose that it takes X days for a to complete alone, then it takes 1.5x days for B to complete alone. Column: 12 (1 / x + 1 / 1.5x) = 130 = 1.5xx = 20

The price of a commodity is increased by 30% according to the cost price, and then it is sold at a 20% discount. The price is 208 yuan. What is the cost price of this commodity______ Yuan

Let the original price be x yuan. According to the equation, we get: X × (1 + 30%) × 80% = 208. The solution is x = 200, that is, the original price is 200 yuan

Express 0.000861 by scientific counting method=_________

0.000861=8.61×10^-4
Do not understand can ask, help please adopt, thank you!

(1 / 2 + 1 / 3 +...) 1 / 2004) x (1 + 1 / 2 + 1 / 2003) - (1 + 1 / 2 + 1 / 2004) x (1 / 2 + 1 / 3 + +1 / 2003)

Let a = 1 / 2 + 1 / 3 + 1 / 2004, B = 1 / 2 + 1 / 3 + +1 / 2003
So the original formula is a (B + 1) - (a + 1) B
=ab+a-ab-b
=a-b
=1 / 2004

As shown in the figure, in square ABCD, e is the point on ad, BF bisection ∠ CBE is called CD at point F

Proof: extend DC to g, make CG = AE, connect BG ∵ square ABCD ∵ AB = BC (the sides of the square are equal) ∵ a = ∵ BCF = 90 ° (the corners of the square are equal and 90 °) ∵ a = ∵ BCG = 90 ° in △ BAE and △ BCG, Ba = BC ∵ a = ∵ bcgae = CG

（5.2-3.7）×8.9+17×0.11

15.22

Fill in the blanks
Because of the popularity of a commodity, the manufacturer decided to reduce the original price by 10%. The current price is 270 yuan, and the original price is x yuan
(1) The reduced selling price can be expressed as:
(2) The countable equation is

(1)x*(1-10%)
(2)x*(1-10%)=270