1. perpendicular

2. What figures are these?

3. What is the angle? Right angle Right angle = 90° “ ” is the symbol for a right angle

4. What is the angle? Acute angle 0°< Acute angle < 90°

5. What is the angle? Obtuse angle 90°< Obtuse angle < 180°

6. What is the angle?. Straight angle Straight angle = 180°

7. What is the angle?. Round angle Round angle = 360°

8. What is this? Set square

9. Right angle Right angle side Right angle side

10. Right angle Right angle side Right angle side

11. two straight lines are intersecting relations. intersection point

12. two straight lines are intersecting relations two straight lines intersect at right angles This is the special case of intersection——perpendicular. when two straight lines intersect at right angles, they are perpendicular to each other.

13. Right angle Right angle side Right angle side In the set square, two right angle sides are perpendicular to each other.

14. a b O Line a and line b intersects at right angle, so line a and line b are perpendicular to each other. line a is perpendicular to line b, write as: a ⊥ b. Or line b is perpendicular to line a, write as: b ⊥ a. ( ⊥ is perpendicular symbol ). O foot of perpendicular

15. Line c and line d intersects at ( ), so line c and line d are ( ). line c is( )line d, write as( ) Or ( ) foot of perpendicular c d E E. right angle perpendicular to each other perpendicular to c⊥dc⊥d line d is perpendicular to line c, write as: d ⊥ c

16. n m Is line m and line n perpendicular to each other? line m and line n are perpendicular to each other. A B C D O Is line AB and line CD perpendicular to each other? line AB and CD is just intersect, not perpendicular to each other.

17. Perpendicular in life The plumb line is always perpendicular with the horizontal plane. So construction workers use plumb lines to check whether the walls are straight or crooked.

18. Perpendicular in life The perpendicular line on the windscreen will indicate the glider is flying direct or not.

19. Judge When two straight lines intersect at 90 °, the two straight lines must be perpendicular to each other. ( ) × √ intersect at 90 ° There is only one line perpendicular to line AB. ( ) A B

20. Judge Two adjacent sides of a square are perpendicular to each other. ( ) × √ adjacent sides of a square When two straight lines intersect ， the intersection point is called foot of perpendicular. ( ) A B C D O intersection point.

21. Look at the figure below, which angle is the right angle? Mark with the right angle symbol. Can you find any perpendicular line segments? Write it down O A B C D A B C D O

22. Thank you!