1, △ = b 2-4ac The discriminant of the root of a quadratic equation of one variable can judge the intersection of the real root of the quadratic equation of one variable and the quadratic function on the X axis. ①△& gt； 0, the equation has two unequal real roots, and the quadratic function has two different intersections with the X axis; ②△=0, the equation has two equal real roots, and the quadratic function intersects with the X axis; ③△& lt； 0, the equation has no real root, and the quadratic function has no intersection with the x axis.

2.√x= open the square of x, let x=9, √x=3.

3. | x | = the absolute value of x, the distance from the point representing x on the number axis to the origin, x = 3, |x|=3.

4.≌ congruence symbol, used to prove triangle congruence in geometric proof, such as △ABC △BCD used for △ ABC and △ BCD congruence.

5.π irrational number Pi is a common symbol in junior high school, indicating numbers, not letters.

6.⊥ Vertical symbols, such as AB⊥CD, indicate that the positional relationship between AB and CD is vertical.

7, trigonometric function symbol sin cos tan. In a right triangle, the opposite side/hypotenuse of sinA = ∠ A, the adjacent side/hypotenuse of cosA =∠A, and the opposite side/adjacent side of Tana = ∠ a.