12

In earlier grades we learnt that in for instance an equilateral triangle all angles are 60 degrees. Now in high school I am taught that measuring angles anti-clockwise and clockwise make a difference.

So does that mean in an equilateral triangle one angle is 60 which is measures anti-clockwise and one is -60 degrees which is clockwise and.. I don't know, I am pretty confused.

How does it add to 180 then?

I was taught about this negative angle concept in trigonometry in unit circles which I understood very well but why is it if not applicable to any geometric figure?

It looks like I am lacking some common sense.

glglgl
  • 103
geek101
  • 1,143
  • 4
    Generally, in geometric figures, we don't care about negative angles for the same reason that we don't care about negative sides. – Alex Silva Dec 29 '14 at 11:00
  • 4
    As Silva said negative angles are not that useful in geometry, but they are widely used in trigonometry and complex algebra where they are expressed in radians – pkjag Dec 29 '14 at 11:02
  • 4
    It's an oriented angle not a geometric angle. – user5402 Dec 29 '14 at 11:08
  • One question...so negative angles are only in trigonometry and complex numbers and no where else in mathematics ?? – geek101 Dec 29 '14 at 11:17
  • 1
    @user166748: Negative angles are used when one wants to distinguish clockwise and anticlockwise. That's all. –  Dec 29 '14 at 16:51
  • @Rahul: abstract sin and tan functions distinguish positive and negative angles, but there is no orientation, only maps from angles to reals. – Incnis Mrsi Dec 29 '14 at 17:15
  • @user166748 We give angles an orientation just as we give line segments an orientation to become vectors. – user5402 Dec 29 '14 at 18:35

5 Answers5

37

The following line segment on the number line has length four.

    ---------
0 1 2 3 4 5 6 7

The following arrow drawn on the number line represents a displacement of $-4$.

    <--------
0 1 2 3 4 5 6 7

The following arrow drawn on the number line represents a displacement of $4$.

    -------->
0 1 2 3 4 5 6 7

You can do the same thing with angles; sometimes you only care about the magnitude, but other times you pick an orientation: you decide which leg of the angle you start at and which leg you end at. When we orient angles, we typically pick counterclockwise to be the positive direction, and clockwise to be the negative direction.

Even more interesting is an idea of angular position versus angular displacement. If we consider an angle whose vertex is the origin and one leg (the starting leg) is the positive $x$ axis, then putting the other leg at $270^\circ$ would be the negative $y$ axis, but $-90^\circ$ would also be the negative $y$ axis.

The idea that $270^\circ$ and $-90^\circ$ represent the same angle is the idea of angular position.

However, there is a difference between actually going counterclockwise $270^\circ$ around the plane from the positive $x$ axis to the negative $y$ axis, and going $90^\circ$ clockwise (i.e. $-90^\circ$ counterclockwise) from the positive $x$ axis to the negative $y$ axis. When this difference matters, we call it "angular displacement". We could even consider going completely around the origin to return to the $x$ axis then continuing another three quarters the way around: this would be a displacement of $630^\circ$!

As an addendum, if we orient the two legs of the angle so that they point in opposite directions in relation to the vertex, then we usually consider the leg pointing away from the vertex to be the starting leg, and the leg pointing towards the vertex to be the ending leg.

This way, if we orient the three sides of a triangle so that the sides all point in a counter-clockwise direction around the triangle, then all of the angles are oriented counter-clockwise too.

  • thanks for the answer..One question...so negative angles are only in trigonometry and complex numbers and no where else in mathematics ?? – geek101 Dec 29 '14 at 11:18
  • 3
    @user166748 Is there another area of mathematics where angles are used? If there is, probably negative angles can be used there, too. – JiK Dec 29 '14 at 12:00
4

In plane geometry, ‘negative angles’ are used when defining angles of a pair of vectors; it's a measurement for the rotation which takes the first vector to the second – so that $ \mathrm{angle}(\vec u,\vec v)=-\mathrm{angle}(\vec v,\vec u)$. As an example in usual life, screwing is not the same as unscrewing.

The difference between geometric angles and ‘algebraic’ angles is similar to the difference betwen a segment and a vector.

Bernard
  • 175,478
  • so negative angles are only in vector algebra.. trigonometry and complex numbers and no where else in mathematics ?? – geek101 Dec 29 '14 at 11:21
  • 1
    You forgot to say these “positive” and “negative” angles make sense only for a plane (two dimensions) with orientation. In 3D angles between vectors are also vectors and haven’t ordering on them. – Incnis Mrsi Dec 29 '14 at 11:23
  • @IncnisMrsi: You're right. Referring to plane geometry was implicit, but it's better to to say it explicitly. – Bernard Dec 29 '14 at 11:49
3

Negative angle measurement don't exist. Sorry.

In a geometric figure like a triangle all angles have positive measurement.

So in a triangle our angles could have measures of $30^o, 60^o, 90^o$ or $\pi/6, \pi/3, \pi/2$ if you like. These are measurements of physical angles which are never negative.

However the measurements are done in degrees or radians which is a numerical system and we can think about a number as $-60^o$ or $-\pi/6$. These are numbers plain and simple.

You probably saw "negative angles" in trignometry class when you were confusing them with negative numbers of angle measurements. Functions like $\sin \theta, \cos \theta, \tan \theta$, etc. are easy to define for $\theta$ between $0^o$ and $360^o$ but we can just as easily define them for any other measurement including numbers like $-60^o$ by realizing that on the unit circle going $-60^o$ (or $60^o$ clockwise) is the same as going $300^o$ counterclockwise.

Hope this helps!

Chinmay Nirkhe
  • 719
  • thanks for the answer @Chinmay Nirkhe- One question...so negative angles are only in trigonometry and complex numbers and no where else in mathematics ?? – geek101 Dec 29 '14 at 11:13
  • 4
    I wonder why so clueless answer defining physical angles in a (mathematical) triangle collected no less than 6 upvotes. It introduces some confusing terminology, but even after that contradicts itself: “In a geometric figure all angles have positive measurement” but “Functions like sinθ, cosθ, tanθ, etc. are easy to define for θ between 0° and 360° but we can just as easily define them for any other measurement including numbers like −60°”. – Incnis Mrsi Dec 29 '14 at 14:58
  • 9
    This answer is bad because it claims that negative angle measurement doesn't exist when it clearly does (relative to a fixed position and axis). This answer explains it better. – Arturo Torres Sánchez Dec 29 '14 at 15:25
  • 2
    Tip: Use ^\circ to make a degree sign: ${}^\circ$. –  Dec 29 '14 at 16:47
  • Think about a sum of vectors from tail to head adding to zero, robot arms travelling in the correct direction to the desired angle, etc. You extract the abs value magnitude from the result when you need it. See Geometric Algebra. Unnecessary left/right handed, counterclock/clockwise conventions end up getting introduced when ambiguity is created by throwing away information. – Rob Dec 29 '14 at 21:38
1

Angles are not negative, however in trigonometry we easily talk about them anyway. The thing we use to make sense of them is modular arithmetic.

Modular arithmetic arranges the numbers in a clock or cycle, for example modular 3 we count $0,1,2,0,1,2,0,1,\dots$. We simply only have $3$ integers, and if we reach $3$ we simply go back to $0$. We therefore say that $3$ modulu $3$ is $0$, which is written as $3\equiv0\pmod{3}$

When using modular numbers, we don't have negative numbers either, and if we go below zero, we end up at the top again, so $-1\equiv2\pmod{3}$

We have a rule here and that is, we can add or subtract the number we're doing the modulus to as many times we want, and it'll remain the same.

When using degrees, a full turn in a circle is $360^\circ$, and when we have performed a full turn in a circle, we're just back where we started. This is just like the modulus operation I talked about above, and we can use the modulus concept to explain how angles operate when going below $0^\circ$ or above $360^\circ$.

When working with degrees, we go back to zero at $360^\circ$, therefore in degrees it makes sense to say that degrees work using modulus $360^\circ$. If we understand angles this way, we can explain what a negative angle really is. Let's for example take $-90^\circ$, we can apply modular arithmetic, which allows us to add $360$ to any angle without changing it: $-90^\circ\equiv270^\circ\pmod{360^\circ}$ and if you draw those two angles on the unit circle, notice how the angles point at the same place.

All this also explains why going above $360^\circ$ allows you to go below again, namely $400^\circ\equiv40^\circ\pmod{360^\circ}$, and therefore the angle $40^\circ$ and $400^\circ$ is just the same thing.

When talking about radians, it's much the same, except a full turn is $2\pi$ instead of $360$, and therefore we have to work modulus $2\pi$ with radians.

Alice Ryhl
  • 7,853
  • I mean, saying something like working with degrees, we're simply working modulus [sic] 360 is a worthless way to explain, because evades definition of the full angle (a.k.a. 1 turn, or revolution) that is necessary to substantiate why should we operate modulo 360 making calculation in degrees and modulo 2π in radians. – Incnis Mrsi Dec 29 '14 at 15:15
  • @IncnisMrsi I have attempted to clarify my answer. – Alice Ryhl Dec 29 '14 at 15:26
  • I retract my downvote in appreciation of cleanup, but do not think this modular arithmetic stuff addresses the question of signedness enough. – Incnis Mrsi Dec 29 '14 at 15:33
0

Angle is a measurement and can't be negative, and it is used to define a certain point on the trigonometric circle. In the trigonometric circle, counterclockwise rotation is denoted the positive sign and counterclockwise is denoted the negative sign. for example, moving 60 degrees in the positive direction is completely identical to moving 300 degrees in the negative direction, as they both define one point. Note that the choice of + or - is completely arbitrary and makes no difference; what really matters is the absolute value of angular displacement.

Mammad Eskan
  • 89
  • 1
    There are several situations where 360°≠0, 540°≠180°, 450°≠90°, and so on. – Incnis Mrsi Dec 29 '14 at 15:25
  • @IncnisMrsi Right. Imagine a screw: from the top, it looks equal no matter if you rotate by 360° or 720°. But the depth of the screw is different. – glglgl Dec 29 '14 at 16:32
  • @IncnisMrsi Of course. in this simple answer I am merely concerned with angular position, and the definition of negative angles. – Mammad Eskan Dec 29 '14 at 22:27