ag-, ig-, act- (Latin: do, act, set in motion, drive, lead, conduct, guide).act: 
activate: 
activation: 
actor: 
actress: 
actuate: 
agenda: 
agent: 
agile: 
agitate: 
ambiguous: 
castigate: 
coact: 
coagulate: 
cogitate: 
congent: 
corrigible: 
enact: 
enactment: 
exact: 
exactness: 
exigency: 
exiguous: 
fumigate: 
fumigation: 
hyperactive: 
ignite: 
immigrate: 
inact: 
inaction: 
inactive: 
interact: 
interaction: 
intransigence: 
intransigent: 
litigate: 
litigation: 
mitigate: 
mitigation: 
navigate: 
navigation: 
proactive: 
radioactive: 
react: 
reaction: 
reactivate: 
retroactive: 
transact: 
transaction: 
Etc. is a perfect word-when you can't think of the right one.
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