Brain Teaser: ImpossibleMath3


Brain teaser:
Reference
[ImpossibleMath3]
Submitter
Jack Tan (jahk@uiuc.edu)
Submission date
14 April 1997

About the ImpossibleMath problems: here are some math problems for various grade levels, from algebra up to calculus. These are strange because the problems give results such as "2 is equal to 1" and "-1 is equal to 1"! Can you find the flaw in the reasoning? (My apologies to those more oriented towards literature and language.)

This problem requires calculus and is intended only for advanced classes. Let the symbol 'Integral(dx, f(x))' stand for the indefinite integral of the function f(x) with respect to x. It might be easier to understand if it is handwritten in traditional notation. Examine the integration of f(x)=1/x with respect to x, integrating by parts. We can show that 0 is equal to 1.

Integrating f(x)=1/x by parts, use u=1/x and dv=dx. Thus:

u=1/x and dv=dx

imply

du= -1/(x^2) * dx and v = x

Putting the pieces back together, we find

Integral(dx, 1/x) = u*v - Integral(v*du)
Integral(dx, 1/x) = (1/x)*(x) - Integral(dx, x*(-1/(x^2)))
Integral(dx, 1/x) = 1 - Integral(dx, -1/x)
Integral(dx, 1/x) = 1 + Integral(dx, 1/x)
Subtract the term 'Integral(dx, 1/x)' from both sides to get:

0 = 1
What is wrong with this reasoning?


Jack Tan and Emmie Chen / jahk@uiuc.edu

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