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© 1999-2010 by Francis Edward Su

From the Fun Fact files, here is a Fun Fact at the Easy level:

# One Equals Zero: Integral Form

Consider the following integral:

INTEGRAL (1/x) dx

Perform integration by parts: let

u = 1/x , dv = dx
du = -1/x2 dx , v = x

Then obtain:

INTEGRAL (1/x) dx = (1/x)*x - INTEGRAL x (-1/x2) dx
= 1 + INTEGRAL (1/x) dx

which implies that 0 = 1.

What's wrong with this calculation?

The Math Behind the Fact:
This is common mistake using integration by parts in calculus. Students often forget about the constant of integration for indefinite integrals. In this case, the constants on both sides will differ by 1.

Su, Francis E., et al. "One Equals Zero: Integral Form." Math Fun Facts. <http://www.math.hmc.edu/funfacts>.

References:

Subjects:    calculus, analysis, other
Level:    Easy
Fun Fact suggested by:   James Baglama
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