Symmedians, Lemoine point
The reflecting of the median over the corresponding angle bisector is the symmedian. The angle formed by the symmedian and the angle bisector has the same measure as the angle between the median and the angle bisector, but it is on the other side of the angle bisector. The symmedian is isogonally conjugate to the median
There are three symmedians. They are meet at a triangle center called the Lemoine point.
Contents
Proportions
Let be given.
Let be the median,
Prove that iff is the symmedian than
Proof
1. Let be the symmedian. So
Similarly
By applying the Law of Sines we get
Similarly,
2.
As point moves along the fixed arc
from
to
, the function
monotonically increases from zero to infinity. This means that there is exactly one point at which the condition is satisfied. In this case, point
lies on the symmedian.
Similarly for point
Corollary
Let be the
symmedian of
Then is the
symmedian of
is the
symmedian of
is the
symmedian of
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Symmedian and tangents
Let and it’s circumcircle
be given.
Tangents to at points
and
intersect at point
Prove that is
symmedian of
Proof
Denote WLOG,
is
symmedian of
Corollary
Let and it’s circumcircle
be given.
Let tangent to at points
intersect line
at point
Let be the tangent to
different from
Then is
symmedian of
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Lemoine point properties
Let be given. Let
be the Lemoine point of
Prove that is the centroid of
Proof
Let be the centroid of
The double area of is
Point is the isogonal conjugate of point
with respect to
Similarly, one can get
The double area of is
Similarly, one can get is the centroid of
Corollary
Vector sum
Each of these vectors is obtained from the triangle side vectors by rotating by and multiplying by a constant
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Lemoine point extreme properties
Lemoine point minimizes the sum of the squares of the distances to the sides of the triangle (among all points internal to
Proof
Let us denote the desired point by Let us imagine that point
is connected to springs of equal stiffness attached to the sides at points
and
and contacts sliding along them without friction. The segments modeling the springs will be perpendicular to the corresponding side. The energy of each spring is proportional to the square of its length. The minimum energy of the system corresponds to the minimum of the sum of the squares of the lengths of these segments, that is, the sum of the squares of the distances from
to the sides.
It is known that the minimum spring energy corresponds to the equilibrium position. The condition of equilibrium at a point is the equality to zero of the vector sum of forces applied from the springs to the point
The force developed by each spring is proportional to its length, that is, the equilibrium condition is that the sum of the vectors
It is clear that the point
corresponds to this condition.
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Lemoine point and perpendicularity
Let be given. Let
be the Lemoine point of
is the midpoint
Prove that
Proof
is isogonal conjugated
with respect
is cyclic.
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Lemoine point line
Let be given. Let
be the Lemoine point of
Let be the height,
be the median,
be the midpoint
.
Prove that the points and
are collinear.
Proof
Denote the circumcenter
Denote the midpoint
is centroid of
is
median of
Denote the point symmetric
with respect
is the midline of
is the median of
is the median of
the points
and
are collinear.
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