Denshi Jisho — Online Japanese dictionary

合う あう 1: to come together; to merge; to unite; to meet;
2: to fit; to match; to suit; to agree with; to be correct;
3: to be profitable; to be equitable;
4: () (after the -masu stem of a verb) to do ... to each other; to do ... together
Common word, Godan verb with u ending, Intransitive verb 
合う おう 1: to come together; to merge; to unite; to meet;
2: to fit; to match; to suit; to agree with; to be correct;
3: to be profitable; to be equitable;
4: () (after the -masu stem of a verb) to do ... to each other; to do ... together
Irregular kana usage, Godan verb with u ending, Intransitive verb 

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