# Unit Circle: an Introduction

By Murray Bourne, 23 Sep 2010

I remember the first time I came across the idea of the "unit circle" and I was quite impressed. There was so much going on!

Let's see what is involved in this very useful bit of mathematics. There are many items of math vocabulary and we can't assume students will know all of them. It would be easy to get lost if you were uncertain of the meaning of just a few of these words. Each mathematical concept is in bold text.

We start with a **point** (call it O) and use it as the **center** of a **circle**, **radius** 1.

Next, we are going to overlay our circle with a **Cartesian coordinate system **(our familiar *x*- and *y*-**axes**), named after the French mathematician who devised it, Rene Descartes.

Next, we add a random point on the circle (0.9, 0.44) and label it P. The numbers in brackets are called the **coordinates of the point** and represent the distance along the *x* and *y*-axes to the point P.

We now draw the radius of the circle passing through the point P and drop the **altitude** from P down to where it meets the *x*-axis at point Q (0.9, 0), forming the **triangle** OPQ. We also measure the **angle** formed by the *x*-axis and the radius OP and it turns out to be 26.37°.

Line PQ has length 0.44 **units** and OQ has length 0.9 units.

Now, we can consider the **trigonometric ratios** involved in this example. The first one is the **sine **of the angle POQ (that is, the ratio of the **length **of the **opposite side **to the length of the **hypotenuse** of the **right triangle**).

Now, this is very neat. It means that in a unit circle, the *y*-value of a point on the circle we are interested in is equal to the sine of the resulting angle at the center.

Now, let's look at the **cosine** case.

Once again we get a neat result. The *x*-value of our point on the circle is equal to the cosine of the angle at the center.

So we could now label point P as (cos 26.37°, sin 26.37°) or using our **variable** for the angle size in this case, P (cos θ, sin θ).

For many students, it's a mystery how we can extend the trigonometric ratios for angles bigger than 90°, and why some of the trig ratios are positive in some quadrants and negative in others.

The unit circle helps us see why that is so. Since the point P is defined as (cos θ, sin θ), where θ is the angle **subtended** at the center, we can find the trigonometric ratios for angles bigger than 90°.

If we move our point P around the circle from the first to the 2nd quadrant, to the point, say (−0.5, 0.87), this is what we get:

This tells me the sine of 120° is 0.87 (the *y*-value) and the cosine of 120° is −0.5 (the *x*-value). (You can check these on your calculator.)

Note that in the first quadrant, both sine and cosine (and therefore tangent) were all positive.

In the second quadrant, sine was positive, cosine was negative (and so tangent would be negative, too).

Recall that

So if sine is positive and cosine is negative, then tangent will be negative.

## Extending the Unit Circle

Of course, we can extend the concepts above for any point and any circle with center (0, 0). All we need to do is change the radius and we can find the trigonometric equivalent for any point.

So for example, if the radius is 8 units, a point on the circle in the 3rd quadrant could be (−2.74, −7.52). The angle this point makes with the positive *x*-axis is 200°.

This time, we have:

8 sin 200° = −2.74 (negative)

8 cos 200° = −7.52 (negative)

Sine and consine are both negative in the 3rd quadrant (so tangent is positive).

## Summary

The unit circle is an interesting concept that ties together several important mathematical ideas, such as Euclidean geometry (circles, points, lines, triangles, etc.), coordinate geometry (the *x-y* plane, coordinates on the plane, etc), and trigonometry (the sine, cosine and tangent ratios).

See the 18 Comments below.

24 Sep 2010 at 12:39 am [Comment permalink]

For an interactive version of the unit circle definitions, there is a java applet (including all six trig functions and their graphs) at

http://qpr.ca/math/applets/trigfuncs/

I hope it's useful and (or) fun for some to play with.

cheers,

Alan

24 Sep 2010 at 6:09 am [Comment permalink]

Thanks, Alan!

6 Oct 2010 at 9:18 pm [Comment permalink]

It is very useful and effective way to use the UNIT circle to explain many things, except the part when you extend the circle and you refer to sin x - 2.73 (a number >1).

thanks

Sam Mazahreh

6 Oct 2010 at 9:32 pm [Comment permalink]

Oops - thanks Sam! I neglected to put the 8 in front of the sin 200° and cos 200°.

I amended the post.

6 Oct 2010 at 9:44 pm [Comment permalink]

This web site is very nice for learning students.

7 Oct 2010 at 1:15 pm [Comment permalink]

This is good

10 Oct 2010 at 3:03 am [Comment permalink]

Thank you very much.

sherly.

11 Oct 2010 at 4:49 pm [Comment permalink]

An Eye-Opener !!

13 Oct 2010 at 12:05 pm [Comment permalink]

It should be angle POQ instead of angle OPQ when you are initially defining the angle in the unit circle.

13 Oct 2010 at 2:24 pm [Comment permalink]

Thanks for pointing out the typo, Rhonda! I fixed it in the post.

16 Oct 2010 at 8:46 pm [Comment permalink]

Tell others join this web to enjoy with mathematical concepts and learn. Thanks Murray

22 Oct 2010 at 7:08 am [Comment permalink]

Lovely explanation. WIll be able to show some of my students this to help clear up some confusion. Thanks

7 Dec 2010 at 10:09 pm [Comment permalink]

Thaqnks amillion for the onfo

16 Jan 2011 at 5:32 am [Comment permalink]

The article is very nice and actually brings out some of the assumptions we teachers often make on the bases of what the students should have learnt in the previous classes. Whatever is to be presented to the students should be determined by the lesson objectives and not what the concept entails.

The unit circle is every useful in determining the trigonometric ratios of integral multiples of 90 degrees. I would have love to submit an article on how I presented the unit circle to my students to enable them use it in finding the trigonometric ratios of integral multiples of 90 degrees but efforts to copy and paste have ended in vain.

16 Jan 2011 at 11:51 am [Comment permalink]

Hi Jaff. I'd like to see your article and I'll ask you to send it to me by email.

18 Jan 2011 at 7:54 pm [Comment permalink]

Jaff sent me the activity sheet he was referring to and here it is: Basic uses of the Unit Circle (MS Word file).

5 Dec 2015 at 4:47 am [Comment permalink]

Hi Murray, you mentioned that unit circle ties together several mathematical ideas e.g. euclidean geometry and trigonometry. Would you please elaborate more on this?

6 Dec 2015 at 6:18 pm [Comment permalink]

@Jessica: The article itself outlined the connection between these 3 branches of mathematics. I amended the final paragraph just now to make it clearer what I meant.