ovo je prvi deo, ima još tri dela - onda ćeti biti jasno , strpljenja
Daj nemoj praviti od sebe budalu. Trisekcija ugla uz pomoc lenjira i sestara je dokazano nemoguca. Razni zaludjenici su stotinama puta postavljali neke svoje "dokaze", koji su svi imali jednu od sledecih osobina:
- nisu bili u skladu sa postavljenim problemom
- nisu bili tacni
- ili oboje.
No ako si vec zapeo postuj svoj "dokaz", objasnicemo ti gde je greska.
Za sad vidim da si prepolovio ugao.
Proof of impossibility
The geometric problem of angle trisection can be related to algebra—specifically, to the problem of finding the roots of a cubic polynomial—since by the triple-angle formula, cos(3θ) = 4cos3(θ) − 3cos(θ).
One can show that any number constructible in one step from a field K is a solution of a second-order polynomial. Note also that π / 3 radians (60 degrees, written 60°) is constructible. We now show that it is impossible to construct a 20° angle; this implies that a 60° angle cannot be trisected, and thus that an arbitrary angle cannot be trisected.
Denote the set of rational numbers by
. If 60° could be trisected, the minimal polynomial of
over
would be of second order. Now let
.
Note that
. Then by the triple-angle formula,
and so 4y3 − 3y − 1 / 2 = 0. Thus 8y3 − 6y − 1 = 0, or equivalently (2y)3 − 3(2y) − 1 = 0. Now substitute x = 2y, so that x3 − 3x − 1 = 0. Let p(x) = x3 − 3x − 1.
The minimal polynomial for x (hence
) is a factor of p(x). Because p(x) is degree 3, if it is reducible over
then it has a rational root. By the rational root theorem, this root must be 1 or −1, but both are clearly not roots. Therefore p(x) is irreducible over
, and the minimal polynomial for
is of degree 3.
So an angle of 60° = (1/3)π radians cannot be trisected.
Many people (who presumably are unaware of the above result, misunderstand it, or incorrectly reject it) have proposed methods of trisecting the general angle. Some of these methods provide reasonable approximations; others (some of which are mentioned below) involve tools not permitted in the classical problem. The mathematician Underwood Dudley has detailed some of these failed attempts in his book The Trisectors.[2]
[edit] Angles which can be trisected
However, some angles can be trisected. For example, for any constructible angle θ, the angle 3θ can be trivially trisected by ignoring the given angle and directly constructing an angle of measure θ. There are also angles which, while non-constructible, are trisectible when given. For example, 3π / 7 is such an angle: five copies of 3π / 7 combine to make an angle of measure 15π / 7, which is a full circle plus the necessary π / 7. More generally, for a positive integer N, an angle of measure 2π / N is trisectible if and only if 3 does not divide N.[3]
[edit] One general theorem
Again, denote the rational numbers
:
Theorem: The angle θ may be trisected if and only if q(t) = 4t3 − 3t − cos(θ) is reducible over the field extension
.
The proof is a relatively straightforward generalization of the proof given above that a 60-degree angle is not trisectible.[4]
Edited by tomas.hokenberi, 12 September 2011 - 19:08.