Let be the nine-point center of triangle
. We showed in our previous post that if the parent triangle is right-angled with
, then
(1)
We’ll now prove what holds more generally for any triangle:
(2)
being the circumradius and
the usual side-lengths.
Memory recall
Let’s first be reminded of the main ingredient: Stewart’s theorem. It states that given a triangle with side-lengths
and a cevian
like the one depicted below:
(3)
Main results
In with the usual notation, PROVE that the orthocenter-circumcenter distance
satisfies
.
It was from this amazing site that we first learnt of the above identity. We’ll add our own proof of it to an already existing pool of proofs, using Stewart’s theorem as a tool. We’ll also use the formula for median lengths, and the fact that the centroid divides a median in the ratio (measured from a vertex) and also divides the Euler line
in the ratio
(measured from the orthocenter).
Consider the diagram below where is the centriod of
,
is the midpoint of
, and
(circumradius):
Notice our new point just
andering along side
. More importantly, observe that
is perpendicular to
(
is the midpoint of
and
is the circumcenter). Therefore, the Pythagorean theorem applied to
gives:
(4)
Since and
, Stewart’s theorem applied to
gives:
(5)
The length of median satisfies
. Thus, combining equations (4) and (5) gives:
The above procedure works regardless of the vertex we started with. And it also works even in the peculiar case where are co-linear. It also doesn’t matter if both
and
are outside the parent triangle.
Let be the orthocenter of
with circum-radius
. PROVE that
.
Apply Stewart’s theorem to in the diagram below:
Add all the three last equations:
Let and
be the nine-point center and orthocenter of
. PROVE that
.
We only slightly modify the proof given in example 2, replacing the centroid with the nine-point center (note that
is the midpoint of
).
In above,
is a median, so:
Similarly, if we connect the segment to vertices
and
instead, we obtain (with
and
as medians)
Add the expressions for and simplify:
Since , we have
. In turn:
PROVE that , where
is the nine-point center of
. Hence deduce that in a right triangle
with hypotenuse of length
, the sum of the squares of the distances from the vertices to the nine-point center is
the length of the hypotenuse.
We obtained as an intermediate step in example 3 above.
Now if is such that
is the hypotenuse with length
, then
. The circumradius
is half of the length of the hypotenuse, so
. Substituting in the expression for
, we obtain:
We show in example 5 below that the above relation also holds in a non-right triangle setting.
Consider in which
and
. PROVE that
.
(If you’re curious as to how we obtained , see example 7 for a sample procedure.)
A key ingredient in the proof is the fact that the circumradius can be expressed as
as per the extended law of sines.
Since , the cosine law gives
Also:
Equation (2) becomes:
Show that the circumradius of an equilateral triangle
can be given by
, where
is the length of one of the sides.
The orthocenter and circumcenter coincide for an equilateral triangle, so the distance . Using example 1 and
, we have:
In , suppose that
. PROVE that either
or
. If one has
in addition, deduce that the triangle is equilateral.
One can easily check that if a right triangle satisfies , then
. So the above example is basically saying that the equation
is not exclusive to right triangles.
From example 2: . Set the left side equal to
:
Thus, either (giving a right triangle), or
as required. Now let , then:
If , then
, which is not allowed for a normal non-degenerate triangle. Therefore we must take
, which yields
. Since we already assumed that
, it follows that we obtain an equilateral triangle.
Mere repetitions
What follows are particular cases of some of the preceding examples, applied to our new point whose coordinates are given by:
(6)
Let be the nine-point center of
and let
be the point given in equation (6). PROVE that
, if
has two sides parallel to the coordinate axes.
Trivial stuff. If has legs parallel to the coordinate axes, then
, based on one of the equivalent conditions in our previous post. The conclusion now follows from example 3.
If has two sides parallel to the coordinate axes, PROVE that
, where
is the point given by equation (6).
Trivial stuff, twice. Under the given condition, we have . Thus, the conclusion follows from example 2 above.
Let be such that two sides have slopes
. PROVE that
, where
is the point with coordinates given by equation (6).
Trivial stuff, thrice. The given triangle is obviously a right triangle. Since the legs have slopes , point
coincides with the circumcenter. Therefore we have
by example 1 above.
Takeaway
Let be the side-lengths of
, and let
be the orthocenter, circumcenter, and nine-point center, respectively.
The following statements are equivalent:
or
The following statements are also equivalent:
or
And the following two statements too are equivalent:
or
Bottom line: Many non-right triangles share properties (relating to and stuff) that one would have thought to be exclusive to the right triangle.
Tasks
- Let
be the centroid of
, and let
be the circumcenter.
- PROVE that
, where
are the usual side-lengths.
- Deduce that
.
- PROVE that
- Let
and
be the orthocenter and nine-point center of
. PROVE that
.
- Let
be the orthocenter, circumcenter and nine-point center of
. PROVE that:
.
- Let
be the orthocenter of
with circumradius
and side-lengths
.
- PROVE that
- Deduce that the distance from vertex
to the orthocenter is twice the distance from the circumcenter to the midpoint of side
.
- PROVE that
- This exercise shows that there are many non-right triangles with the property that the length of the Euler line
is half of the length of a side of the triangle. Suppose that in
one has
. PROVE that:
or
- if
in addition, then
. (Thus, any isosceles triangle in which
and
will satisfy
. And it’s even possible to realize this with a non-right, scalene triangle.)
- Consider
with vertices at
,
,
. Verify that:
- the orthocenter is
and the circumcenter is
- the orthocenter is
- Find coordinates for the vertices of a non-right triangle
with orthocenter
and side-lengths
such that
.
- Find coordinates for the vertices of a non-right triangle
with nine-point center
and side-lengths
such that
.
- Consider a quadrilateral
with vertices at
,
,
,
. PROVE that:
- sides
and
have opposite slopes, and their lengths are in the ratio
- sides
and
have opposite slopes, and their lengths are in the ratio
- diagonals
and
have opposite slopes, and their lengths are in the ratio
- the longer diagonal
bisects the shorter diagonal
- the shorter diagonal
divides the longer diagonal
in the ratio
is a cyclic quadrilateral.
(The pointthat partly facilitated these features is among the quartet of points we’ll introduce later this year, if time/space permits.)
- sides
- Consider
with vertices at
,
,
,
. PROVE that
, where
is the length of the median from vertex
.