# Why does a circles angles equal up to 2pi? Wouldn't it be 1?

My geometry teacher (last year - and now he's gone) drew a circle for me and defined all the points in terms of pi (is there an ASCII character for pi, by-the-way, on the keyboard?). Where the 90 degree point would be he put 1/2 pi, were the 180 degree point was he put just 'pi', at 270 he put 3/2 pi, and 360 he put 2 pi. What the heck?! I thought pi was the ratio of the diameter to the circumference. If that is true then shouldn't it be: at 90 degrees 1/4 pi, at 180 deg. 1/2 pi, 270 deg. 3/4 pi, and 360 deg. just 'pi'? Can you explain this or direct me to a site that can? Please, I'm going crazy trying to figure it out.

Last edited:

Robert Zaleski
Therefore, 2pi radians would be 2 x 57.2958 x 3.1416 = 360 degrees

Janus
Staff Emeritus
Gold Member

Originally posted by Cyberice
My geometry teacher (last year - and now he's gone) drew a circle for me and defined all the points in terms of pi (is there an ASCII character for pi, by-the-way, on the keyboard?). Where the 90 degree point would be he put 1/2 pi, were the 180 degree point was he put just 'pi', at 270 he put 3/2 pi, and 360 he put 2 pi. What the heck?! I thought pi was the ratio of the diameter to the circumference. If that is true then shouldn't it be: at 90 degrees 1/4 pi, at 180 deg. 1/2 pi, 270 deg. 3/4 pi, and 360 deg. just 'pi'? Can you explain this or direct me to a site that can? Please, I'm going crazy trying to figure it out.
What your geometry teacher was doing was marking out the circles in Radians.

1 Radian is the angle that an arc the same length as the radius of the circle will make.

And since, as you pointed out, the ratio of diameter to circumference is [pi], and the radius is 1/2 the diameter, there are 2[pi] radians to a circle.

Thus a quarter of the way around the circle is 1/2 [pi] radians, half way [pi] radians, three quarters, 1 1/2 [pi] radians and all the way around 2[pi] radians.

Radians are the natural way to divide up a circle. ( the number of degrees were just an arbitary choice)

Thank you! I was wondering about that and it really bothered me. BTW, would you happen to know any sites that could explain it also (just for the heck of it)?

My answer is so that the angle times the radius gives the arc length.

Doesn't that mean 2 Pi radians is a calculation convenience?

Integral
Staff Emeritus
Gold Member
The number 2&pi; is forced on us, it is the circumfrence divided by the radius. Now, Janus says this in his post but I will repeat it. Take the radius, mark that length off on the arc of the circumference, the angle defined is 1 radian. This definition means that there must be 2&pi; radians in a circle. Repeat that to yourself until it starts to make sense.

Ah..., no, I really do NOT miss Donde!

FZ+
To add, the use of this natural units system makes alot of calculations possible. Eg. calculus with trignometric functions, which is generally based on the result that: x -> 0 : sinx -> x, provided x is in radians.

Using a numberline helped me out tremendously.

Euler is the first person who started to use radian in trigonometry. The circumference of a unit circle is 2pi, therefore he defined 360 degrees be 2pi radian.

A radian is the measure of an angle subtended at the centre of a circle by an arc equal in length to the radian.

einsteinhawking
(URGENT) What does subtended mean?

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What does subtended mean? I forgot and I have a test tomorrow

Thank-you It makes it clearer although I already had my test