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Tag: triangle geometry

Steiner Inellipse: A Surprising Area Characterization
Given a triangle, there are many ellipses that can be drawn inside it. Among them, one has a particularly beautiful and distinguished place in geometry: the Steiner inellipse.

What is the Steiner inellipse?

Let ABC be any triangle. The Steiner inellipse is the unique ellipse contained in the triangle that is tangent to the three sides at their midpoints.

Thus, if D, E, and F are the midpoints of BC, CA, and AB, respectively, the Steiner inellipse passes through all three points and is tangent to the corresponding sides there.

Its center is the centroid G of the triangle—the point where the three medians intersect.

The Steiner inellipse also has an extremal property: among all ellipses contained in a given triangle, it has the largest possible area.

Why does the Steiner inellipse naturally appear?

One way to understand the Steiner inellipse is through affine geometry. Every triangle can be obtained from an equilateral triangle by an invertible affine transformation.

For an equilateral triangle, the Steiner inellipse is simply its incircle. The incircle touches the three sides at their midpoints and is centered at the common centroid, incenter, and circumcenter.

Under an affine transformation, a circle generally becomes an ellipse, midpoints remain midpoints, and tangency is preserved. Therefore the incircle of an equilateral triangle is transformed into an ellipse tangent to the three sides of the new triangle at their midpoints. That ellipse is precisely the Steiner inellipse.

So the Steiner inellipse may be viewed as the affine image of the incircle of an equilateral triangle.

Figure 1. The Steiner inellipse of triangle ABC.
It is the unique ellipse tangent to the three sides at their midpoints,
and its center is the centroid G.

A different way to recognize the same ellipse

The definition above characterizes the Steiner inellipse by tangency: it touches the sides of the triangle at three special points.

But there is another, rather unexpected way to detect whether a point lies on this ellipse—one that does not initially mention an ellipse, tangency, or even distances.

It uses only parallel lines and areas.

Choose an arbitrary point M inside triangle ABC. Through M, draw three lines, each parallel to one side of the triangle.

These three lines cut off three smaller triangles at the vertices A, B, and C. Let their areas be

$T_{1}, T_{2}, T_{3} .$

Let $T$ denote the area of the original triangle.

Now ask a simple question:

For which points M is the sum of the three corner areas exactly one-half of the area of the original triangle?

$T_{1} + T_{2} + T_{3} = \frac{T}{2} ?$

At first glance, there is no obvious reason that the answer should involve an ellipse at all.

Figure 2. Through an arbitrary interior point M,
draw three lines parallel to the sides of triangle ABC.
The three corner triangles have areas T₁, T₂, and T₃.

The surprising answer

The answer is remarkably simple: the points satisfying this area condition are exactly the points on the Steiner inellipse.

Theorem. Let ABC be a triangle with area $T$ , and let M be a point in its interior. Through M, draw three lines parallel to the sides of the triangle, cutting off three corner triangles with areas $T_{1}$ , $T_{2}$ , and $T_{3}$ . Then
$M \in Steiner inellipse \Leftrightarrow T_{1} + T_{2} + T_{3} = \frac{T}{2} .$

In other words, a point M lies on the Steiner inellipse if and only if the three corner triangles together have exactly half the area of the original triangle.

This is unexpected because the construction itself contains no ellipse. We choose a point, draw three parallel lines, and measure three areas. Yet the condition that their sum equals one-half of the total area traces out precisely the Steiner inellipse.

Why should this be an ellipse?

The key is affine geometry. An invertible affine transformation sends triangles to triangles, preserves parallelism and ratios of areas, and sends ellipses to ellipses.

We can therefore transform our original triangle into an equilateral triangle without changing the essential area condition. In an equilateral triangle, the Steiner inellipse becomes something much more familiar: the incircle.

So it is enough to determine which points satisfy the area condition in the equilateral case.

Figure 3. A point M on the Steiner inellipse.
For this point, the three corner areas satisfy T₁ + T₂ + T₃ = T/2.

The equilateral case

Consider the equilateral triangle with vertices
A=(−1,0), B=(0,3), C=(1,0).
The base has length 2 and the height is $\sqrt{3}$ , so the area of the triangle is
T = 2·3 2 = 3.
Let $M = (a, b)$ be an interior point. The three lines through M parallel to the sides cut off three smaller triangles. Because each corner triangle is similar to the original equilateral triangle, their areas can be written in terms of a and b.

A direct calculation gives
T1 = 34 ( 1 +a − b3 ) 2 , T2 = 34 ( 1 −a − b3 ) 2 ,
and
T3 = b2 3 .
Now impose our area condition:
T1 + T2 + T3 = T2 = 32.
Substituting the three expressions above and simplifying gives
a2 + b2 − 2b 3 = 0.
Completing the square transforms this into

$a^{2} + {(b - \frac{1}{\sqrt{3}})}^{2} = \frac{1}{3} .$

But this is the equation of the circle centered at
( 0, 13 )
with radius $\frac{1}{\sqrt{3}} .$ This is precisely the incircle of our equilateral triangle.

Therefore, in the equilateral case, the points satisfying
T1 + T2 + T3 = T2
are exactly the points on the incircle.

Equilateral triangles often turn geometric questions into especially elegant problems. For another example, see Equilateral Triangle Maximum Area .

Figure 4. In the equilateral case, the Steiner inellipse is the incircle. The area condition produces a circle centered at (0, 1/√3) with radius 1/√3.

Returning to the original triangle

We have proved that, for an equilateral triangle, the condition

$T_{1} + T_{2} + T_{3} = \frac{T}{2}$

describes exactly the incircle.

Now apply the inverse affine transformation that carries the equilateral triangle back to the original triangle. Parallel lines remain parallel, and all areas are multiplied by the same factor, so the area condition is preserved. The incircle is transformed into the Steiner inellipse.

Therefore, for any triangle, a point M satisfies the area condition if and only if M lies on the Steiner inellipse.

A related area identity

There is another elegant relation involving the same three corner triangles. Unlike the characterization above, this identity holds for every interior point M, not only for points on the Steiner inellipse.

Since each corner triangle is similar to the original triangle, the ratio of corresponding side lengths is the square root of the ratio of the corresponding areas. The three relevant length ratios add to 1, which gives

$\sqrt{T_{1}} + \sqrt{T_{2}} + \sqrt{T_{3}} = \sqrt{T} .$

The contrast between the two identities is worth noticing.

For every interior point M,

$\sqrt{T_{1}} + \sqrt{T_{2}} + \sqrt{T_{3}} = \sqrt{T} .$

But without the square roots,

$T_{1} + T_{2} + T_{3} = \frac{T}{2}$

holds precisely when M lies on the Steiner inellipse. Thus the same three corner areas give both a universal identity and a geometric characterization of a special ellipse.

There is another remarkable way in which the Steiner inellipse appears. If the vertices of the triangle are regarded as the three complex roots of a cubic polynomial, the zeros of its derivative are exactly the two foci of the Steiner inellipse. See Marden’s Theorem: How the Derivative of a Cubic Finds an Ellipse .

References and further reading
1. A. Eydelzon, “On a New Property of the Steiner Inellipse” , The American Mathematical Monthly, Vol. 127, No. 10 (2020), pp. 933–935.
2. NCS/MAA Team Contest, Thirteenth Annual Contest (2009), Problem 9 , “Square roots of area ratios.”
3. D. Kalman, “An Elementary Proof of Marden’s Theorem” , The American Mathematical Monthly, Vol. 115, No. 4 (2008), pp. 330–338.
The first reference contains the area characterization of the Steiner inellipse discussed in this article. The second gives an earlier appearance of the classical square-root area problem. The third provides additional background on the Steiner inellipse and its connection with Marden’s theorem.

A triangle is a two-dimensional simplex, while a tetrahedron is its three-dimensional counterpart. For a related exploration of simplex geometry, see The Geometry of a Tetrahedron: From Pythagoras to Vector Identities .
October 7, 2026
Can a Circle Cut a Triangle in Half?

Orange is inside the circle; blue is outside. Each occupies half the triangle.

Take a triangle $ABC$ . Place a compass point at $A$ , draw a circle, and adjust its radius until half the triangle lies inside the circle. What does the answer look like?

For an equilateral triangle of side $1$ , the calculation is pleasantly short. The portion inside the circle is a sector with angle $\pi/3$ . Since the triangle has area $\sqrt3/4$ , we want

$\frac{\pi r^2}{6}=\frac{\sqrt3}{8}, \qquad\text{hence}\qquad r=\sqrt{\frac{3\sqrt3}{4\pi}}.$

The circle has not yet reached the opposite side $BC$ , so the sector calculation really does describe the part of the triangle inside the circle. A vertex-centered circular arc bisecting an equilateral triangle also appears in Sanjoy Mahajan’s 2008 MIT course materials.

Now change the triangle. The circle might meet $BC$ before it captures half the area. Or it might grow past one of the other vertices. The figure above shows the three possible pictures.

A radius always exists

As the radius increases from $0$ , the area of the triangle inside the circle increases continuously from $0$ to the area of the whole triangle. It increases strictly until the farther vertex is reached. Therefore exactly one radius divides the triangle into equal areas.

We can scale the triangle without changing which of the three pictures occurs. To simplify the calculations, label the vertices so that $AC\le AB$ , and set $AC=1$ . Write $A,B,C$ for the angles, measured in radians. By the sine rule,

$AB=L=\frac{\sin C}{\sin B}, \qquad T=\operatorname{area}(ABC)=\frac{L\sin A}{2}.$

The desired area inside the circle is $T/2$ .

Case 1: A sector is enough

While the circle remains inside the triangle’s angle at $A$ , its area inside the triangle is $Ar^2/2$ . The candidate radius is thus

$\boxed{r_0=\sqrt{\frac{T}{A}}=\sqrt{\frac{L\sin A}{2A}}.}$

We still need to check whether the circle reaches $BC$ . If both base angles are acute, the distance from $A$ to $BC$ is $h=\sin C$ , so this is the answer precisely when $r_0\le h$ . The test can be written entirely in angles:

$\boxed{\cot B+\cot C\le 2A.}$

If $C$ is right or obtuse, the nearest point of the segment $BC$ to $A$ is $C$ . In that situation the sector answer works when $r_0\le AC=1$ .

Case 2: The circle crosses $BC$ twice

Suppose both base angles are acute, $h<r<1$ , and the circle cuts $BC$ at two points. Start with a sector and subtract the circular segment lying beyond $BC$ :

$S(r)=\frac{Ar^2}{2} -r^2\arccos\!\left(\frac{h}{r}\right) +h\sqrt{r^2-h^2}, \qquad h=\sin C.$

The equal-area radius is the unique solution of $S(r)=T/2$ in $(h,1)$ . We can determine before solving whether it lies there. At $r=1$ ,

$S(1)=\frac{C-B}{2}+\sin C\cos C.$

If the sector candidate already exceeds $h$ , and $S(1)\ge T/2$ , we are in this case. Equality gives the boundary case $r=1$ .

For example, a triangle with $A=150^\circ$ and $B=C=15^\circ$ has an equal-area radius of approximately $0.335593$ when $AC=1$ . Its circle crosses $BC$ twice.

Case 3: The circle passes a vertex

If half the area has still not been captured at $r=1$ , the circle passes the nearer vertex $C$ . The remaining part outside the circle sits near $B$ , and $1<r<L$ .

Let $D$ be the circle’s intersection with $BC$ , and set $\phi=\angle BAD$ . The sine rule in triangle $ABD$ gives

$\phi=\arcsin\!\left(\frac{\sin C}{r}\right)-B.$

The outside area equals the area of triangle $ABD$ minus a sector centered at $A$ :

$U(r)=\frac{Lr\sin\phi-r^2\phi}{2}.$

Set $U(r)=T/2$ , or equivalently $Lr\sin\phi-r^2\phi=T$ . This determines the unique radius between $1$ and $L$ . As an example, $A=80^\circ, B=20^\circ, C=80^\circ$ gives $r\approx1.014031$ when $AC=1$ .

The equilateral triangle leads to a square-root formula. Other triangles lead to a circular segment, and some require us to measure the outside area instead. The same question changes character as the angles change.

Another Equal-Area Problem

Here the challenge was to use a circle to divide the area of a triangle exactly in half. What happens if we reverse the roles and ask a circle to divide the area of another circle in half? Surprisingly, we can even require the center of the cutting circle to lie outside the original circle.

Continue exploring: Can a Circle Centered Outside Another Circle Cut Its Area Exactly in Half?

September 23, 2026

Tag: triangle geometry

Steiner Inellipse: A Surprising Area Characterization

What is the Steiner inellipse?

Why does the Steiner inellipse naturally appear?

A different way to recognize the same ellipse

The surprising answer

Why should this be an ellipse?

The equilateral case

Returning to the original triangle

A related area identity

References and further reading

Can a Circle Cut a Triangle in Half?

A radius always exists

Case 1: A sector is enough

Case 2: The circle crosses BCBC twice

Case 3: The circle passes a vertex

Another Equal-Area Problem

Case 2: The circle crosses $BC$ twice