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 .

- 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 .

- 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

- 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 point that partly facilitated these features is among the quartet of points we’ll introduce later this year, if time/space permits.)

- Consider with vertices at , , , . PROVE that , where is the length of the median from vertex .