[計量] 一題pp

[surfingdream (GMATing)]

If the operation @ is defined for all integers a and b by a@b=a+b-ab,which of
the following statements must be true for all integers a,b and c

III)(a@b)@c =a@(b@c)

請問一下上面這個怎麼算出來是true
感謝


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