ヤク 
つづ.まる つづ.める つづま.やか 
(動) promise (形) approximately (動) 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