1. quantities with opposite meanings
In real life, there are some words with opposite meanings, which reflect some different situations, states or processes, such as "up and down" and "expansion". When these words are combined with numbers and units, they constitute quantities with opposite meanings, such as "up 0. 1 yuan" and "out 80t". This concept includes:
(1) has opposite meanings, such as east and west, income and expenditure, etc.
(2) It is the same quantity, such as "10 meter higher than 300 yuan" is not the inverse quantity.
2. Positive and negative numbers
(1) positive number: such as+1, +3/2,+1.05, etc. The numbers learned in these primary schools are all preceded by "+".
This symbol is a positive number. At this time, "+"does not represent addition, but represents the nature of numbers. For example, "+1" is pronounced as "positive 1", and the "+"before the positive number can be omitted.
Car spray 7
(2) Negative numbers: such as-1, -7/3, -2. 1, etc. A number preceded by a "-"is a negative number.
Represents the nature of a number, pronounced as "negative number", and the "-"sign before the negative number cannot be omitted.
(3) About the meaning of "0".
0 is neither a positive number nor a negative number, but a "dividing line" between positive and negative numbers. At the same time, it is no longer the number that means "no" as understood by primary schools, and it is no longer the smallest number. Combining with real life has its own significance, such as "00C" indicating freezing point temperature.
3. Use positive numbers and negative numbers to represent quantities with opposite meanings.
Positive numbers are numbers greater than 0, and negative numbers are numbers less than 0. Positive and negative numbers can be used to represent these quantities with opposite meanings in life. There are many quantities with opposite meanings in nature, such as rising 5 meters and falling 6 meters, 8 kilometers east of l0, a surplus of 654.38+10,000 yuan, a loss of 20,000 yuan and so on. , which can be represented by positive and negative numbers.
Guide to solving problems
The following quantities with opposite meanings are represented by positive numbers and negative numbers.
(1) In the knowledge contest, if+10 means adding 10 points, how to deduct 20 points?
(2) When someone turns the turntable, if +5 turns indicate 5 turns counterclockwise, what is the representation of 12 turns clockwise?
(3) In a ping-pong quality test, what does it mean that a ping-pong ball exceeds the standard weight by +0.02g and is recorded as +0.02g and -0.03g?
Analysis: (1) The meanings of plus points and minus points are opposite, and+10 means plus 10 points, so the deduction of 20 points should be represented by -20;
(2) Rotating the dial counterclockwise and clockwise indicates the opposite meaning. If the counterclockwise rotation is positive, the clockwise rotation is negative;
(3) The quantity of quality exceeding the standard in the opposite sense is lower than the standard quality, where 0.02g of quality exceeding the standard means +0.02g, and -0.03g means 0.03g of quality below the standard.
Solution: (1) Deduct 20 points and record it as-20;
(2) Turn clockwise 12 turn, and record it as-12 turn;
(3)-0.03g means that the table tennis is 0.03g below the standard mass.
Note: Two quantities with opposite meanings stipulate that one quantity is represented by a positive number and the other quantity is represented by a negative number. It is only a rule to express a quantity with a positive number or a negative number, but it often follows people's habits. For example, people are used to expressing temperatures above zero with a positive number and income with a positive number.