GWD 23-6

[Apple.chang]

The operation @ is defined by x@y=1/x+1/y for all nonzero numbers x andy. If z is a number greater than 1, which of the following must be true?

I. x@(-z)=0
II. z@z/z-1=1
III.2/z@2/z=z

A. I only
B. I and II only
C. I and III only
D. II and III only
E. I,II, and III

answer: D

請問本題怎麼解?
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[Apple.chang]

想到怎麼解了...不好意思喔