Angle between points

[DrStyles]

Is there any way to find the angle between two points without trig functions?

 

[Vellu]

Angle of two points? Isn't that 180, since it's a straight line between them. But if you want to know an angle of a triangle's one corner, I think the only way is to use trigonometric functions. Or you can always measure the angle using a triangle ruler?

 

[ElderKingpin]

180*, and a triangle ruler is called a protractor, i could be wrong though, i dont know how many strange things are used to do math ._.

I think it would help if you were less vague, two points could be anywhere and anything

 

[Moridin]

I think he means on an X, Y plane...

 

[ElderKingpin]

thats still really vague

 

[13lade619]

there are no angles in cartesian / pure algebra.

that's what trigonometry is for.

 

[DrStyles]

I think he means on an X, Y plane...

Ya, that's what I meant. I have the x,y of both points and I need to find the angle without trig functions, but I don't think it's possible. >.>

 

[13lade619]

finding the slope is the closest thing you can get.

but you have to do trigonometry to convert that to an angle in degrees.

 

[UnknowVector]

First, why don't you want to use trigs?

I'm assuming you either want the angle >ABC where A and C are your points and B is the origin, or one of the angles AC makes with either the x or y axis when it crosses them. Either of those will require trig, specifically the use of the inverse tangent function.

The Maclaurin series (http://en.wikipedia.org/wiki/Taylor_series)

x - x^2 / 2 + x^3 / 3 - x^4 / 4 + x^5 / 5 ...

provides a passable approximation of arctan(x) from -.305 to .359 (less than 0.1 margin of error), so if you know the slopes of the lines AB and BC are between those values, you can use that series instead of arctan in the formula for >ABC, and if you know the slope of AC is in that range, then you can use the series in the the formulas for the angles formed by AC, the x-axis, and the y-axis. That assumes you know enough trig. to figure out the formula you are approximating though!

Continuing that series will yield a better estimate near x=0, but if your slopes aren't nearly zero, your out of luck.

So, really, no. There's no way without trig.