Inverse Pythagorean theorem

Relation between the side lengths and altitude of a right triangle From Wikipedia, the free encyclopedia

Inverse Pythagorean theorem

In geometry, the inverse Pythagorean theorem (also known as the reciprocal Pythagorean theorem[1] or the upside down Pythagorean theorem[2]) is as follows:[3]

Let A, B be the endpoints of the hypotenuse of a right triangle ABC. Let D be the foot of a perpendicular dropped from C, the vertex of the right angle, to the hypotenuse. Then
Comparison of the inverse Pythagorean theorem with the Pythagorean theorem using the smallest positive integer inverse-Pythagorean triple in the table below.
More information BasePytha-gorean triple, AC ...
Base
Pytha-
gorean
triple
ACBCCDAB
(3, 4, 5)20 = 515 = 512 = 425 = 52
(5, 12, 13)156 = 12×1365 = 5×1360 = 5×12169 = 132
(8, 15, 17)255 = 15×17136 = 8×17120 = 8×15289 = 172
(7, 24, 25)600 = 24×25175 = 7×25168 = 7×24625 = 252
(20, 21, 29)609 = 21×29580 = 20×29420 = 20×21841 = 292
All positive integer primitive inverse-Pythagorean triples having up to three digits, with the hypotenuse for comparison
Close

This theorem should not be confused with proposition 48 in book 1 of Euclid's Elements, the converse of the Pythagorean theorem, which states that if the square on one side of a triangle is equal to the sum of the squares on the other two sides then the other two sides contain a right angle.

Proof

Summarize
Perspective

The area of triangle ABC can be expressed in terms of either AC and BC, or AB and CD:

given CD > 0, AC > 0 and BC > 0.

Using the Pythagorean theorem,

as above.

Note in particular:

Special case of the cruciform curve

Summarize
Perspective

The cruciform curve or cross curve is a quartic plane curve given by the equation

where the two parameters determining the shape of the curve, a and b are each CD.

Substituting x with AC and y with BC gives

Inverse-Pythagorean triples can be generated using integer parameters t and u as follows.[4]

Application

If two identical lamps are placed at A and B, the theorem and the inverse-square law imply that the light intensity at C is the same as when a single lamp is placed at D.

See also

References

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