Task list filter?
Choose required ranks and required tasks. The table of contents will list only tasks having one of the required ranks in corresponding rankings and at least one of the required tags (overall). If you wish to filter only according to some rankings or tags, leave the other groups empty.
Task rankings
Difficulty level
Task tags
Task type
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Product with -1
Task number: 2483
Show that in any field K it holds that ∀a∈K:(−1)a=−a.
Resolution
(−1)a=(−1)a+0=(−1)a+(a−a)=((−1)a+1a)−a=(−1+1)a−a=0a−a=−a.
