Lesson Explainer: Angles in Standard Position | Nagwa Lesson Explainer: Angles in Standard Position | Nagwa

Lesson Explainer: Angles in Standard Position Mathematics

In this explainer, we will learn how to identify angles in standard position and find the positive and negative measures of their equivalent angles.

Each of the angles that we will look at will be graphed on the coordinate plane. The coordinate plane is formed by the intersection of a horizontal line, which we refer to as the 𝑥-axis, and a vertical line, which we refer to as the 𝑦-axis. The point of intersection of the two lines is called the origin. The two axes divide the coordinate plane into four regions, which we call quadrants. We label the quadrants in a counterclockwise direction with roman numerals as follows.

Whether or not an angle is in standard position on the coordinate plane depends upon the position of one of the rays that make up the angle and on the position of the angle’s vertex.

Definition: Angle in Standard Position

An angle is in standard position if its vertex is at the origin on the coordinate plane and if one ray is coincident with the positive 𝑥-axis. The ray that is coincident with the positive 𝑥-axis is called the initial side of the angle, and it is where the angle is measured from. The other ray is called the terminal side.

An example of an angle in standard position is 𝐴𝑂𝐵 in the figure below.

We can see that its initial side, 𝑂𝐴, is coincident with the positive 𝑥-axis or the part of the 𝑥-axis to the right of the origin. We can also see that its vertex, point 𝑂, is at the origin.

The measure of an angle is the amount of rotation from its initial side to its terminal side and can be given in degrees or radians. One full revolution of the terminal side is equivalent to 360 or 2𝜋 radians. If an angle is measured in a counterclockwise direction, we classify it as positive, and if it is measured in a clockwise direction, we classify it as negative.

We can use the fact that the measures of angles in the same direction around a point sum to 360 to find a negative angle with the same terminal side as a given positive angle and a positive angle with the same terminal side as a given negative angle. These pairs of angles are called coterminal angles.

Definition: Coterminal Angles

Coterminal angles are angles in standard position on the coordinate plane that have the same terminal side.

Every positive angle has a negative coterminal angle, and every negative angle has a positive coterminal angle. The angles measuring 90 and 270 in standard position are examples of coterminal angles, because each of their terminal sides lies on the positive 𝑦-axis as follows.

These angles are also referred to as quadrantal angles, as is any angle in standard position that has its terminal side on one of the axes of the coordinate plane.

The angles measuring 60 and 420 in standard position are other examples of coterminal angles, because their terminal sides are in the same position relative to the positive 𝑥-axis. In other words, the angles have the same terminal side.

Notice that the measure of the 420 angle is 360 more than the measure of the 60 angle. We can add any multiple of 360 to an angle to produce the measure of one of its coterminal angles.

Let’s begin our examination of angles with an example in which we must decide if a given angle is in standard position.

Example 1: Identifying Angles in Standard Position

Is the angle in standard position?

Answer

Recall that an angle is in standard position if its vertex is at the origin on the coordinate plane and one ray is coincident with the positive 𝑥-axis. The ray that lies on the positive 𝑥-axis is called the initial side of the angle, and the other ray is called the terminal side.

Notice that the angle in the figure is being measured in a counterclockwise direction. We can see that one of the rays that makes up the angle, or the angle’s initial side, does indeed lie on the positive 𝑥-axis. We can also see that the other ray that makes up the angle, or the angle’s terminal side, is located in quadrant I of the coordinate plane. The point where the two rays meet, or the vertex of the angle, lies at the origin.

Thus, we can say that, yes, the angle in the figure is in standard position.

In the last example, we looked at an angle in standard position. Let’s compare the angle in that example with the one in the figure below.

Here, we can see that although the vertex of the angle is at the origin on the coordinate plane, the angle’s initial side lies not on the positive 𝑥-axis, but on the negative 𝑥-axis. Therefore, had this been the figure we were given in the example, we would have said that, no, the angle is not in standard position.

The following angle is another angle that is not in standard position.

Here, although the vertex of the angle is at the origin on the coordinate plane, the angle is being measured from a side that is not on the positive 𝑥-axis. This is indicated by the direction of the arrow in the figure. Had the arrow pointed in the opposite direction, the angle would have been in standard position.

Now let’s look at another angle and determine if it is in standard position.

Example 2: Determining if an Angle is in Standard Position

Is the angle in standard position?

Answer

Remember that if the vertex of an angle is at the origin of the coordinate plane, and if one ray, referred to as the initial side, is coincident with the positive 𝑥-axis, then the angle is said to be in standard position. The position of the other ray, referred to as the terminal side, does not matter.

In the figure we have been given, the angle is being measured in a counterclockwise direction. The angle’s initial side lies on the positive 𝑥-axis of the coordinate plane, while the other ray, or the angle’s terminal side, lies in quadrant IV. Also, the vertex, or the point where the two rays meet, lies at the origin.

Therefore, we can say that, yes, the angle in the figure is in standard position.

Note

Suppose that, instead, we had been given the following figure.

In this figure, our angle is being measured in a clockwise direction, which means that its measure is not positive, but negative. However, the initial side of the angle is again on the positive 𝑥-axis of the coordinate plane, and the vertex is again at the origin, so had the figure appeared this way, we would still have said that, yes, the angle is in standard position.

In our next example, we will determine if an angle expressed as an ordered pair is in standard position.

Example 3: Understanding if an Angle is in Standard Position

Does the ordered pair 𝐶𝐴,𝐶𝐷 express an angle in standard position?

Answer

We know that for the ordered pair 𝐶𝐴,𝐶𝐷 to express an angle in standard position, the angle the ordered pair represents would have to have its initial side coincident with the positive 𝑥-axis of the coordinate plane and its vertex at the origin. Since 𝐶𝐴 is given first in the ordered pair, it is the initial side of our angle. We can see that this ray lies entirely on the positive 𝑥-axis.

It meets the terminal side, 𝐶𝐷, of our angle at point 𝐶, the angle’s vertex. This point is also on the positive 𝑥-axis of the coordinate plane but not at the origin.

Since the vertex is not at the origin, we can say that, no, the ordered pair 𝐶𝐴,𝐶𝐷 does not express an angle in standard position.

Note

Although the angle expressed by the ordered pair 𝐶𝐴,𝐶𝐷 is not in standard position, we can identify multiple angles in the figure that are in standard position. One is given by the ordered pair 𝑂𝐶,𝑂𝐸, another is given by the ordered pair 𝑂𝐴,𝑂𝐺, and a third is given by the ordered pair 𝑂𝐶,𝑂𝐵. For each of these angles, the initial side is on the positive 𝑥-axis, and the vertex, 𝑂, is at the origin.

We should also consider the angle given by the ordered pair 𝑂𝐸,𝑂𝐺. The vertex, 𝑂, of this angle is at the origin, but the angle’s initial side, 𝑂𝐸, is not on the positive 𝑥-axis. Thus, we know that this angle is not in standard position.

The example that follows asks us to find a positive coterminal angle for a given angle with a negative measure. Recall that coterminal angles in standard position on the coordinate plane share the same terminal side.

Example 4: Finding Coterminal Angles

State the smallest positive angle that is equivalent to the angle shown.

Answer

We know that the 340 angle shown is in standard position because its initial side is coincident with the positive 𝑥-axis and its vertex is at the origin. We can also see that this angle is being measured in a clockwise direction, which is why the measure of the angle is a negative number of degrees.

The smallest positive angle that is equivalent to the angle shown is being measured in a counterclockwise direction. We know that it is also in standard position because it has the same initial side and vertex as the 340 angle. It also has the same terminal side as the 340 angle, which means that this angle and the 340 angle are coterminal angles.

Since the negative sign in the measure of the 340 angle just tells us the direction in which the angle is begin measured, let’s ignore it for the time being and take the measure of this angle to be 340. Remember that the measures of angles in the same direction around a point sum to 360, so by subtracting 340 from 360, we can find the measure of the smallest positive angle that is equivalent to the angle shown to be 360340=20.

Next, not only will we find a negative coterminal angle for a given angle with a positive measure, but we will find a positive coterminal angle as well.

Example 5: Identifying Multiple Equivalent Coterminal Angles

Find a positive and a negative coterminal angle for 340.

Answer

Recall that coterminal angles are angles in standard position on the coordinate plane that have the same terminal side. Let’s begin by finding a negative coterminal angle for 340.

Remember that positive angles in standard position on the coordinate plane are measured in a counterclockwise direction, while negative angles are measured in a clockwise direction. We can use this fact, along with the fact that coterminal angles share the same terminal side, to draw the 340 angle and a negative coterminal angle as shown below.

Notice that both angles share an initial side on the positive 𝑥-axis and a vertex at the origin. Recall that the measures of angles circling a point sum to 360, so to find the measure of a negative coterminal angle for 340, we can begin by subtracting 340 from 360 to get 360340=20.

Now, since the negative coterminal angle is being measured in a clockwise direction, we can say that its measure is 20.

Next, let’s find the measure of a positive coterminal angle for 340. To do so, we will consider the fact that one full turn about a point measures 360. If, after rotating the terminal side of an angle in standard position on a coordinate plane 340 counterclockwise about the origin, we were to then rotate it 360 more degrees about the origin in the counterclockwise direction, our terminal side would be in the same position it was before the additional 360 rotation. In other words, the resulting angle would be coterminal with the 340 angle. This is shown in the following two figures.

Thus, to find the measure of a positive coterminal angle for 340, we can add 360 to 340 to get 340+360=700.

In summary, we can say that a positive coterminal angle for 340 is 700, and a negative coterminal angle for 340 is 20.

In the last example, we found a positive and a negative coterminal angle for 340. It is worth noting that since we can fully rotate the terminal side of an angle in standard position on the coordinate plane about the origin any number of times, 700 and 20 are not the only positive and a negative coterminal angles for 340. To find some additional positive coterminal angles, we could continue to add 360 to get 340+360+360=1060,340+360+360+360=1420,340+360+360+360+360=1780, and so on. To find some additional negative coterminal angles, we could continue to subtract 360 to get 20360=380,20360360=740,20360360360=1100, and so on. There are actually an infinite number of positive and a negative coterminal angles for 340, or indeed, any angle.

As a final example, we will look at a problem in which we are given an angle and are asked to determine the quadrant of the coordinate plane in which it lies.

Example 6: Determining the Quadrant in Which a Given Angle Lies

In which quadrant does the angle 242 lie?

Answer

Remember that the coordinate plane is split into four quarters, called quadrants. We label the quadrants in a counterclockwise direction with roman numerals. The upper-right quadrant is called quadrant I, the upper-left quadrant is called quadrant II, the bottom-left quadrant is called quadrant III, and the bottom-right quadrant is called quadrant IV, as shown in the following diagram.

The quadrant an angle lies in is the quadrant in which its terminal side lies when the angle is in standard position. Therefore, to find the quadrant in which the 242 angle lies, we will need to sketch the angle in standard position.

We know that the initial side of any angle in standard position must be coincident with the positive 𝑥-axis, and its vertex must be at the origin. We also know that the angle is being measured in a clockwise direction, because its measure is a negative number of degrees.

Since there are 360 in a full turn about the origin, we can calculate the number of degrees in a quarter turn to be 3604=90. This means that after rotating an angle’s terminal side 90 clockwise from the positive 𝑥-axis, it would lie on the negative 𝑦-axis, giving us a quadrantal angle measuring 90.

Likewise, after rotating it 180 clockwise, it would lie on the negative 𝑥-axis, giving us a quadrantal angle measuring 180, and after rotating it 270 clockwise, it would lie on the positive 𝑦-axis, giving us a quadrantal angle measuring 270. Rotating the terminal side 360 clockwise from the positive 𝑥-axis would put it back on the positive 𝑥-axis, giving us a quadrantal angle measuring 360, as shown in the figure.

This tells us that

  • any angle measuring between 0 and 90 would have its terminal side in quadrant IV,
  • any angle measuring between 90 and 180 would have its terminal side in quadrant III,
  • any angle measuring between 180 and 270 would have its terminal side in quadrant II,
  • any angle measuring between 270 and 360 would have its terminal side in quadrant I.

An angle measuring 242 would hence appear as follows.

Thus, since 242 is between 180 and 270, we know that the angle lies in quadrant II or the second quadrant.

Now let’s finish by recapping some key points.

Key Points

  • An angle is in standard position on the coordinate plane if its vertex is at the origin and one ray is coincident with the positive 𝑥-axis.
  • For an angle in standard position on the coordinate plane, the ray that lies on the positive 𝑥-axis is called the initial side of the angle, and it is where the angle is measured from. The other ray is called the terminal side.
  • An angle on the coordinate plane that is measured in a counterclockwise direction is positive, while an angle measured in a clockwise direction is negative.
  • Coterminal angles are angles in standard position that share the same terminal side.
  • Every angle has an infinite number of positive and negative coterminal angles. These can be found by adding integer multiples of 360 to the angle or subtracting them from the angle.
  • If the terminal side of an angle in standard position on the coordinate plane lies on one of the axes, the angle is called a quadrantal angle.

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