ヤク 
つづ.まる つづ.める つづま.やか 
() promise () approximately () abbreviate
                     
やく (adv) (1) approximately, about, (n) (2) promise, (3) shortening, reduction, simplification, (4) (ling) contraction (in phonetics)
やっかん (n) agreement, stipulation, article, clause
やくそく (n,vs) (1) arrangement, promise, appointment, pact, engagement, (2) convention, rule
やくそくどおり (n,adv) as one promised

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Etymologies: 中文字譜 : 説文解字