Lagrange Multipliers Bash (ha!)
by math_explorer, Sep 19, 2010, 5:43 AM
Quote:
Part 1/1136
Suppose the vertices of the Miquel's triangle are
,
,
. We want to minimize the area of
, which we split into
,
,
.
Let's first consider
.
Let
,
,
. The triangle's area is equal to
. Note that, since
are concyclic,
and
are supplementary, so
. Therefore, the area of
is minimized when
is minimized, which is when
and
are the feet of the perpendiculars dropped from
to
and
.
This condition is symmetric and satisfies the Miquel's triangle condition, so the area of
is minimized at the same time the areas of the triangles
and
are minimized, which is the same time the area of the big triangle
is minimized.
Apparently this triangle is called the pedal triangle of
. Okay, that's the answer.







Let's first consider

Let















This condition is symmetric and satisfies the Miquel's triangle condition, so the area of




Apparently this triangle is called the pedal triangle of

Part 2/1138
Assume the same variable and point names from Part 1. Let
,
,
. The area we want to maximize is
![\[ \sum_{\text{cyc}} (1/2) de \sin \angle C \]](//latex.artofproblemsolving.com/c/e/8/ce8ae4d048f3b51517233e8403053040fb2eda15.png)
(abuse of cyclic sum notation here). By the Law of Sines, that is maximized when this expression is maximized:
![\[ \sum_{\text{cyc}} cde \]](//latex.artofproblemsolving.com/a/2/a/a2a9c2ae1a4b4823b83e0f433a04d550a994f79d.png)
Let's switch variables:
,
,
(barycentric coordinates normalized to sum to the area of the triangle). The constraint is
, where
is the area of
. The expression we want to maximize can be rewritten as
![\[ \frac{1}{abc} \sum_{\text{cyc}} c^2 xy \]](//latex.artofproblemsolving.com/6/f/6/6f6210f112516df58b6375c09f3e418d3f4c544a.png)
We drop the outside fraction and bash to get
![\[ z = \frac{s(c^2)(c^2 - a^2 - b^2)}{a^4 + b^4 + c^4 - 2a^2b^2 - 2a^2c^2 - 2b^2c^2} \]](//latex.artofproblemsolving.com/2/0/0/2003b261daec793550b9b9a85746d8d62fff9849.png)
and symmetric expressions for
and
. This turns out to be proportional to
(Law of Cosines, and note that the denominator is symmetric), which becomes trilinear coordinates
, so
is the circumcenter.



![\[ \sum_{\text{cyc}} (1/2) de \sin \angle C \]](http://latex.artofproblemsolving.com/c/e/8/ce8ae4d048f3b51517233e8403053040fb2eda15.png)
(abuse of cyclic sum notation here). By the Law of Sines, that is maximized when this expression is maximized:
![\[ \sum_{\text{cyc}} cde \]](http://latex.artofproblemsolving.com/a/2/a/a2a9c2ae1a4b4823b83e0f433a04d550a994f79d.png)
Let's switch variables:






![\[ \frac{1}{abc} \sum_{\text{cyc}} c^2 xy \]](http://latex.artofproblemsolving.com/6/f/6/6f6210f112516df58b6375c09f3e418d3f4c544a.png)
We drop the outside fraction and bash to get
![\[ z = \frac{s(c^2)(c^2 - a^2 - b^2)}{a^4 + b^4 + c^4 - 2a^2b^2 - 2a^2c^2 - 2b^2c^2} \]](http://latex.artofproblemsolving.com/2/0/0/2003b261daec793550b9b9a85746d8d62fff9849.png)
and symmetric expressions for





The calculus trap is a terrible thing.
This does not count as a solution.
This post has been edited 1 time. Last edited by math_explorer, Aug 19, 2011, 9:43 AM