选修二英语单词表人教版黑体（学科英语---数学）

第二课Angle Basics

right angles 直角

acute angles 锐角

obtuse angles 钝角

straight angle 平角

complementary angle 余角

supplementary angle 补交

Today we’re gonna learn about angles.In our last video,we learned about points and lines.That’s good because we’re gonna need lines to make angles. So let’s start with a couple of lines that are on the same plane. We’re only going to deal with two dimensional geometry in this video.

These lines are conveniently called “line AB” and “line CD”. Now the important things to notice about these two lines is that they are pointing at exactly the same direction. So even if we extend them forever,they will never cross or even get closer together.

When two lines are arranged like this,we call them “parallel”. You probably heard the term parallel before like parallel parking,parallel universe or parallel bars.

So parallel lines are lines that would never cross even if they go on forever. But if I take one of our lines and give it a little push. Now the lines are not parallel anymore. In fact they cross at this point right here,let’s name it point P.

When lines cross at a point like this,we say that they’re intersect and we call the point an intersection.

And when lines intersect, they form angles. You can think of the angles as spaces or shapes that are form between the intersecting lines. These intersecting lines form four angles:1,2,3 and 4.

But instead of calling them angles one two three and four,in geometry we name them by the points used to make them. For example,this angle here can be called “angle DPB” because if you trace along those points like connect the dots, they outline that angle.

Now in naming angles,there’s a short hand that we can use. Instead of writing “angle” over and over again, we can just use angle symbol instead,which looks like this.

There are three kinds of main angles that you need to know about:right angles,acute angles and obtuse angles.

Actually there’s one more type of angle that is truly important. But it’s kind of a strange one. It’s called “the straight angle”.

There’s just a few more geometry terms that we need to learn in this video.

Let’s look at our simple right angle again that’s made from two rays. But this time let’s draw a third ray that cuts that right angle into two smaller parts. Now because the angle we divided up is a right angle, we know that two smaller angles combined to form the right angle. And in geometry,any two angles that combined to form a right angle are called “complementary angles”.

And we can do the same thing with the straight angles. If we take a straight made from two rays,and divided it with a third ray, two new smaller angles are formed. And those angles combined to form a straight angle,we call these angles “supplementary angles”.

So complementary angles combine to form right angles and supplementary angles combine to form straight angles.