According to R.J. Cook & G.V. Wood (2004). Feynman's Triangle. *Mathematical Gazette* 88:299-302, this triangle result puzzled famous physicist Richard Feynman in a dinner conversation:
For a triangle *ABC* in the plane, if each vertex is joined to the point (1/3) along the opposite side (measured say anti-clockwise), then area *ABC* = 7 x area *UWV* (the inner triangle formed by these lines).

Feynman's Triangle

**Challenge**

Can you prove the above result? Can you prove it in more than one way?

For an elementary proof, easily accessible at secondary school level, see the visual proof by Steinhaus (1960) at Wikipedia in the references below.

**Investigate Further**

1. Can you generalize to find a formula for a triangle *ABC* in the plane, when each vertex is joined to the point (1/*p*), along the opposite side (measured say anti-clockwise)?

2. In the dynamic sketch above, click on the button '**Link to ... general investigate 1**' and use the sliders to investigate Question 1 above. After investigating, click on the next '**Link to ... general formula 1**' button to check your solution.

3. Can you generalize even further if the 3 sides of the triangle are each divided into different ratios? Click on the '**Link to ... Routh formula**" to check your answer. For an elementary proof of Routh's Theorem (1896), easily accessible at secondary school level, see Y.K. Man (2009) in the references below.

**Parallelogram Generalization**

4. Can you generalize the Feynman result for a triangle to a parallelogram?

5. Have a look at *Feynman Parallelogram Generalization* and use the slider for *p* to investigate Question 4 above.

**Investigate Variations**

Also investigate the following variations for a triangle and a parallelogram - use the '**Link**' buttons to navigate from the one variation to the other: *Feynman variations*.

**Bibliography & Related Links (according to date)**

De Villiers, M. (2005). Feedback: Feynman's Triangle (extended). *The Mathematical Gazette*, 89 (514), March, p. 107.

Todd, P. (2006). Feynman's and Steiner's triangle. *The Journal of Symbolic Geometry*, pp. 85-90.

Clarke, R.J. (2007). A generalisation of Feynman's triangle. *The Mathematical Gazette*, Vol. 91, No. 521 (Jul.), pp. 321-326.

Man, Y.K. (2009). On Feynman's Triangle problem and the Routh Theorem. *Teaching Mathematics & its Applications*, 28(1):16-20.

Hindin, H.J. (2010). From Feynman to Fibonacci And More. Paper presented at the Fifth Annual Spuyten Duyvil Mathematics Conference, St. Francis College, Brooklyn, New York, April 24, 2010.

Lingefjärd, T. (2015). Learning Mathematics through Geometrical Inquiry. *At Right Angles*, March, Vol. 4, No. 1, pp. 50-55.

Wikipedia (last edit 13 July 2021). One-seventh area triangle.

Nestor Sánchez León. (31 July 2021). Generalización del triángulo de Feynman, an interactive *GeoGebra* page.

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Created by Michael de Villiers, Sept 2009; modified 26 July 2021; updated 1 August 2021.