I wrote a couple of posts a while back on the Barning Tree method of finding Pythagorean triples using matrices, and then a followup. I recently came across another approach to Pythagorean triples that involves a clever relationship between points on the positive y-axis, and points on the unit circle. (ok, maybe I should have known this, but I didn't.. and I think it is really a neat idea)

On the unit circle, x

^{2}+ y

^{2}= 1, if we draw a secant from the point (-1,0) through (0,t) on the y-axis, it turns out that if t is a rational number, then the coordinates of P=(x,y) where the secant intersects the circle, will also be rational. Since the slope of the line is also t, the equation is y=tx+t ... and so and t

^{2}= (1-x

^{2})/(1+x)

^{2}.

That means t = y/(x+1) which leads to x= (1-t^2)/(1+t^2) and y=

^{(2t)}/_{(1+t^2)}If we pick some rational number to be t, say t=2/7,then x= ^{45}/_{53} and y= ^{28}/_{53}.... Then by similar triangles, there must be a circle with radius 53 and a point on the circle would be (28, 45) and in fact 28^{2}+45^{2}=53^{2}... and any such rational point will produce another.

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