Key issues in the second volume of the first day of junior high school
Among the following surveys, () is suitable for a comprehensive survey (census).
A. visual acuity survey of 50 students in a class
B. Investigation on the quality of dumplings in the market during the Lantern Festival
C. quality investigation of fireworks and firecrackers.
D. Investigation on water quality of Jialing River in Chongqing
Solution: A, the eyesight survey of 50 students in a class is relatively easy to do and suitable for a comprehensive survey, so this option is correct;
B. The investigation on the quality of dumplings in the market during the Lantern Festival is extensive, difficult to do and not suitable for comprehensive investigation, so this option is wrong;
C, the quality of a certain kind of fireworks, destructive investigation, can only use sampling survey, so this option is wrong;
D. The water quality of Jialing River in Chongqing can't be surveyed, and only sampling survey can be used, so this option is wrong.
So choose: a.
Key knowledge points of mathematics
One and only one straight line is perpendicular to the known straight line. Of all the line segments connecting a point outside the straight line with a point on the straight line, the vertical line segment is the shortest (in short, the vertical line segment is the shortest).
Only one straight line passes through two points, and the line segment between the two points is the shortest.
Complementary angle: the sum of two angles is 90 degrees, and these two angles are called complementary angles.
Complementary angle: the sum of two angles is 180 degrees, and these two angles are called complementary angles.
Vertex relative: two angles have a common vertex, and two sides of one angle are opposite to the extension lines of two sides of the other angle. These two angles are antipodal angles.
Isomorphism angle: in the "three-line octagon", the angles at the same position are isomorphic angles.
Internal dislocation angle: in the "three-line octagon", the angle sandwiched between two straight lines is the internal dislocation angle.
Inner angle on the same side: in the "three-line octagon", it is sandwiched between two straight lines, and the angle on the same side as the third straight line is the inner angle on the same side.
Important knowledge points of junior one mathematics
After passing a point outside the straight line, there is one and only one straight line parallel to this straight line.
If both lines are parallel to the third line, then the two lines are also parallel to each other.
Condition of parallel lines: two lines are cut by a third line. If congruent angles are equal, two straight lines are parallel.
Two straight lines are cut by a third straight line. If the internal angles are equal, two straight lines are parallel.
Two straight lines are cut by a third straight line. If they are complementary, then these two straight lines are parallel.
The above are the key math problems I summarized for you in the second volume of Grade One for reference only.