Use Fubini. Then, pause. Then, ask yourself why the argument doesn't work, in general, if $X$ can take both positive and negative values.
– cardinalOct 18 '13 at 20:01
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See discussion of the result here and here and here. I've only heard it expressed as 'the expectation is the integral of the survival function' ... rather than with any particular name.
– Glen_bOct 18 '13 at 20:04
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Incidentally, for continuous random variables, you can show it in two lines using integration by parts.
– Glen_bOct 18 '13 at 20:14