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Negative math problems in the first day of junior high school
(1) Let A, B, C and D be rational numbers that are not equal to 0. Try to explain that at least one of the four numbers-AB, -bc, -cd and da takes a negative value and at least one takes a positive value.

Four numbers, five situations

1. All four are positive numbers.

Then, at least one of the four numbers -ab, -bc, -cd and da is negative and at least one is positive.

2. One of the four is negative, assuming a (the other three assumptions are the same).

Then, -ab is positive, -bc is negative, -cd is negative and da is negative. The conclusion is valid.

3. two are negative numbers, assuming a, b,

Then, -ab negative number, -bc positive number, -cd negative number and da negative number hold.

4. Three negative numbers, assuming that A, B and C are all negative numbers.

Then, -ab negative number, -bc negative number, -cd positive number and da negative number hold.

5. All negative numbers

Then, -ab negative number, -bc negative number, -cd negative number and da positive number hold.

So the conclusion is 1 The following statements are correct: (a) "5m to the east" and "westward10m" are not quantities with opposite meanings; B. If the balloon rises 25 meters and is marked as +25 meters, then-15 meters means falling-15 meters; C. If the temperature drops by 6℃ and is recorded as -6℃, then +8℃ means 8℃ above zero; D. If the height of 1 m is set as standard 0 and the height of 1.20 m is set as +0.20 m, the height of -0.05 m is 0.95 m. 2. Indicate the numbers in the following questions with positive or negative numbers: (1) If the train departs 400 km eastward, it is marked as. (3) If-40,000 means a loss of 40,000 yuan, then a surplus of 30,000 yuan is recorded as _ _ _ _ _ _; (4)+ 150m means altitude 150m, and below 200m, it should be marked as _ _ _ _ _ _; 3.( 1) If the northward direction is positive, what is the significance of walking 3.5km,-1.2km and 0 km? (2) The temperature at noon 12 is 20℃, which is 4℃ higher than that at noon 12 at 2 pm and 5℃ lower than that at 8 pm. What is the temperature at two o'clock in the afternoon? What's the temperature at 8 pm? Answer: 1. According to the analysis, A, B, C and D are all wrong, and only D is right. ∴ Answer: D.2. (1)-4000 km; (2) negative 2 innings; (3)+30,000 yuan; (4)-200 meters. 3.( 1) 3.5 kilometers is 3.5 kilometers to the north; To-1.2km is to the south1.2km; Walking 0 km means standing still. (2) The temperature at 2pm is: 20+4 = 24 (℃), and the temperature at 8pm is: 20-5 = 15 (℃).