Algebraic expression: an expression formed by connecting numbers or letters representing numbers with operational symbols is called an algebraic expression, and a single number or letter is also an algebraic expression. Divisibility of numbers: the characteristics of numbers divisible by 2 are even numbers; The characteristic of numbers divisible by 3 is that the sum of numbers on each digit can be divisible by 3; A number divisible by 4, characterized in that the last two digits are multiples of 4; The characteristic of a number divisible by 8 is that the last three digits are multiples of 8.

Prime number and composite number: integers greater than 1 If there are no other factors except 1 and itself, such an integer is called a prime number, and 1 is neither a prime number nor a composite number. An integer greater than 1 is called a composite number if there are other factors besides 1 and itself. Arithmetic square root: Generally speaking, if the square of a positive number X equals A, that is, X? =a, then this positive number x is called the arithmetic square root of a.

Geometry: Geometry includes line segments, angles, triangles, rectangles, circles, etc. Intersection: When two straight lines intersect and one of the four angles is a right angle, the two straight lines are perpendicular to each other. One of the straight lines is called the perpendicular of the other straight line, and their intersection point is called the vertical foot.

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Parallel lines: In the same plane, two disjoint lines are called parallel lines. Shortest vertical line segment: The vertical line segment is the shortest of all the line segments connecting the points outside the line and the points on the line. Simply put: the vertical line is the shortest.

9. Distance from point to straight line: The length from a point outside the straight line to the vertical section of this straight line is called the distance from point to straight line. Concept of angle: An angle is a graph composed of two rays with a common endpoint.

Angle measurement: Angle can be measured with a protractor, usually in degrees. Classification of angles: angles can be divided into acute angles, right angles, obtuse angles, right angles and rounded corners. Concept of verticality: If two straight lines intersect at right angles, then the two straight lines are perpendicular to each other. Concept of parallel lines: In the same plane, two lines that do not intersect are called parallel lines.