The meaning and reading method of decimal 1 are written on the right side of the integer bit and separated by dots, which is called decimal. 2. To the left of the decimal point is its integer part, and to the right is its decimal part. 3. When writing decimals, the integer part is written as an integer, the decimal point is written in the lower right corner of each digit, and the decimal part is written on each digit in turn. Comparison of the nature and size of decimals 1, add "0" or remove "0" at the end of decimals, and the size of decimals remains unchanged. This is the nature of the so-called decimal. 2. Compare the sizes of two decimals, first look at their integer parts, and the number with larger integer parts will be larger; If the integer parts are the same, the tenth largest number is larger; One tenth of the numbers are the same, and the number with the largest number in the percentile is even larger ... 3. Move the decimal point one place to the right, and the original number will be expanded by 10 times; If the decimal point is moved two places to the right, the original number will be expanded by 100 times; ..... 4. If the decimal point is shifted to the left by one place, the original number will be reduced by 10 times; If the decimal point is moved two places to the left, the original number will be reduced by 100 times; ..... "Addition and subtraction of decimals" 1, calculation rules of decimal addition and subtraction: to calculate decimal addition and subtraction, first align the decimal point of each number (that is, align the numbers on the same digit), then calculate according to the rules of integer addition and subtraction, and finally align the decimal point on the horizontal line of the obtained number. There is a 0 at the end of the decimal part of the number. Generally, 0 should be deleted. The measurement of angle 1, the straight line is infinite. 2. A section between two points of a straight line is called a line segment. A line segment has two endpoints. A line segment is a part of a straight line. 3. Extend one end of the line indefinitely, and you will get a ray. The figure composed of two rays drawn from a point is called an angle. This point is called the vertex of the angle. These two rays are called the edges of the angle. 5. First overlap the vertices of two corners with one edge, and then look at the position of the other edge. Which corner has the other side outside, which corner is big. If the other side also coincides, the two angles are equal. 6. Divide the semicircle into 180 equal parts, and the angle of each part is called 1 degree angle. 7. The size of the angle depends on the size of both sides. The bigger the fork, the bigger the angle. The size of the angle has nothing to do with the length drawn on both sides of the angle. 8. The right angle is 90 degrees. 9. Two sides of an angle form a straight line, and such an angle is called a straight angle. A right angle is 180 degrees. 10, 1 boxer =2 right angles. 1 1 and an angle less than 90 degrees is called an acute angle; An angle greater than 90 degrees and less than 180 degrees is called an obtuse angle. 12. The angle formed by the rotation of light around its endpoint is called fillet. The fillet is 360 degrees. 13, 1 rounded corner =2 flat angle =4 right angle "vertical and parallel" 1. When two straight lines intersect at right angles, they are said to be perpendicular to each other, one of them is said to be perpendicular to the other, and the intersection of these two straight lines is called vertical foot. 2. The length of the vertical line segment drawn from a point outside the straight line is called the distance from that point to the straight line. 3. Two straight lines that don't intersect in the same plane are called parallel lines. It can also be said that these two straight lines are parallel to each other. "Triangle" 1, the figure surrounded by three line segments is called triangle. Each line segment constituting a triangle is called an edge of the triangle, and the intersection of every two line segments is called the vertex of the triangle. 2. The triangle is very stable. 3. A triangle with three acute angles is called an acute triangle; A triangle with a right angle is called a right triangle; A triangle with an obtuse angle is called an obtuse triangle. A triangle with two equal sides is called an isosceles triangle. In an isosceles triangle, the equal sides are called waist; The other side is called the bottom; The included angle between the two waists is called the vertex angle; The two corners on the bottom edge are called bottom corners. A triangle with three equilateral sides is called equilateral triangle, also called regular triangle. Draw a vertical line from the vertex of a triangle to its opposite side. The line segment between the vertex and the vertical foot is called the height of the triangle, and the opposite side is called the bottom of the triangle. 6. The sum of the internal angles of a triangle is 180 degrees. "parallelogram and trapezoid" 1, the figure surrounded by four line segments is called quadrilateral. 2. Two groups of parallelograms with parallel opposite sides are called parallelograms. 3. Parallelogram is easy to deform. 4. Draw a vertical line from one point on one side of the parallelogram to the other. The line segment between this point and the vertical foot is called the height of the parallelogram, and the opposite side is called the bottom of the parallelogram. 5. The two opposite sides of a rectangle and a square are parallel, so they can be regarded as special parallelograms. Source: (/s/blog _ 4df63af90100ch30.html)-Mathematical Concepts in the Second Volume of Grade Four _ Pistachio _ Sina Blog 6. A quadrilateral with only one set of parallel sides is called a trapezoid. 7. In a trapezoid, a group of mutually parallel opposite sides are called the bottom of the trapezoid (usually the shorter bottom is called the upper bottom and the longer bottom is called the lower bottom); A group of non-parallel opposite sides is called trapezoidal waist; Draw a vertical line from a point on the upper bottom to the lower bottom, and the line segment between this point and the vertical foot is called the height of the trapezoid. 8. An isosceles trapezoid is called an isosceles trapezoid. Decimal times decimal 1. To calculate decimal multiplication, first calculate the product according to the law of integer multiplication, and then look at a factor * * *, how many decimal places there are, count from the right side of the product and point to the decimal point. 2. Two factors * * * have several decimals, and the product also has several decimals. When the multiplier ratio is 1, the product ratio is smaller than the multiplicand, and when the multiplier ratio is greater than 1, the product is larger. "Decimal divided by integer" 1, the divisor is the decimal division of integer, which is removed according to the law of integer division, and the decimal point of quotient should be aligned with the decimal point of dividend; If there is a remainder at the end of the dividend, add 0 after the remainder and continue the division. 2. Divide a decimal by an integer, and calculate according to the decimal division calculation rule that the divisor is an integer. If the quotient obtained by division is not enough to get the quotient 1, write 0 in this bit. 3. Divider is the division of decimals. First, move the decimal point of the divisor to make it an integer, move the decimal point of the divisor to the right by several digits, and then move the decimal point of the dividend to the right by several digits, and then calculate according to the division of the divisor as an integer. "Approximation of quotient" is decimal division. When calculating the approximate value of quotient, it is generally necessary to divide by one more decimal point to be retained, and then remove the last digit by "rounding method". Circular Decimal and Understanding 1, to judge whether it is a circular decimal. According to the meaning of circular decimal, the decimal part must be one or several numbers that appear repeatedly in turn, and such a decimal is a circular decimal. 2. The cyclic decimal is infinite, and the cyclic segments are repeated in turn, so the number of cyclic segments is infinite. 3. The cycle segment from the first digit of the decimal part is called pure cycle decimal. The part of the cycle that does not start with the first decimal part is called mixed cycle decimal. 4. When two numbers are divided, the divisible quotient is a finite decimal and the divisible quotient is a cyclic decimal. 5. In the process of decimal division calculation, if you encounter a circular decimal, you can take its approximate value as needed. When the abbreviated cyclic decimal is approximated, it can be rewritten into the original form, and then approximated according to the requirements of "rounding" method; The zero after the decimal point after approximation cannot be removed casually, and attention should be paid to the use of equal sign and approximate equal sign. Simple Equation 1, write the algorithm expressed by letters: additive commutative law writes: a+b=b+a additive associative law: (a+b)+c=a+(b+c) multiplicative commutative law: a×b=b×a multiplicative associative law: (A× B )× C. Square: perimeter = side length ×4. Area = side length × side length parallelogram: area = bottom × height. Triangle: area = base × height ÷2. Trapezoid: area = (upper bottom+lower bottom) height ÷2. 3. The relationship between equations is that equations are equations, equations are not necessarily equations, and equations contain unknowns. 4. The value of the unknown that makes the left and right sides of the equation equal is called the solution of the equation. 5. Find the value of the unknown quantity in the equation, that is, find the solution of the equation. The process of finding the solution of an equation is called "solving the equation". 6. "The solution of the equation" refers to the value of the unknown, which is a number. 7. "Solving equation" is the calculation process of the value of knowledge number X.8.. The relationship between the known number and the obtained number in the four operations: minuend = difference+Mei Mei = minuend-difference one factor = product ÷ dividend another factor = quotient × divisor = dividend ÷ quotient.