386 MATHEMATICSMAGAZINE In the revolutionof a graphy = f(xl) aboutthe y-axis, we let F(l X2, X3) = f (x + X2 + X) . Using sphericalcoordinates the integralbecomes I jlv/1 + (f/(p))2 p2 sin 0 dpdqds= 47rp2 + ( (p))2p dO O a a Formula(4) follows. For revolution aboutthe x-axis, the surface y2 + y2 + y2 = f2(X) can be parame- terizedusing sphericalcoordinates as x = t, Y = f(t) sin cos , Y2 = f(t) sin sin0, 3 = f(t) cos for a < t < b, 0 < 0 < r, 0 < 0 < 2r. Formula(4) follows aftera straightforward calculationof the determinantin (5). Epilogue Now thatwe have extendedthe resultsto fourdimensions, we mightcon- sider extendingthem furtherto n dimensions.In fact, in higherdimensions there are otherways to revolvegraphs about axes. For example,in four dimensionswe could considerthe doublerotation, where the graphis firstrevolved about the y-axis in three dimensionsand thenthe resultingsurface is revolvedabout the x-axis in four dimen- sions. REFERENCES 1. E. A. Abbott,Flatland, Harperand Row Publishers,NY, 1983. 2. D. Davis, TheNature and Power of Mathematics,Princeton University Press, Princeton,NJ, 1993. 3. J. H. Hubbardand B. B. Hubbard,Vector Calculus, Linear Algebra, and Differential Forms, Prentice Hall, Upper Saddle River,NJ, 1999. 4. G. J. Porter,k-volume in ln and the GeneralizedPythagorean Theorem, Amer. Math. Monthly 103 (1996), 252-256. A VectorApproach to Ptolemy'sTheorem ERWIN JUST NORMAN SCHAUMBERGER Bronx Community College of the City University of New York Bronx, NY 10453
[email protected] Ptolemy'sTheorem [1] statesthat the productof the diagonalsof a cyclic quadrilateral (a quadrilateralthat can be inscribedin a circle) is equalto the sum of the productsof its oppositesides.