Word Roots
1. ac, acu = sharp, bitter
acrid: sharp or unpleasant taste or smell
acumen: sharpness of mind; good judgment
acute: severe, intense, sharp-witted
acerbic: sharp and forthright, especially in tone
acrimonious: bitter or sharp in behavior or speech
Note: "ac-" sometimes means "to, toward" (from the prefix "ad-" before 'c'), as in accept, accede, accentuate, acclimate
2. ag, act (also ig) = to do, to drive
agent: someone who acts for another
activate: to make active
react: to respond to something
interact: to act together
counteract: to act against, reduce an effect
agile: able to move quickly and easily
agitate: to stir up or disturb
agency: an organization that acts for others
agility: ability to move quickly/easily
navigate: to direct or move through an area
litigate: to carry on a lawsuit
castigate: to criticize or punish severely
exigent: requiring immediate action or attention
3. acro = top, height, tip, beginning
acrobat: someone who performs high or skillful feats
acronym: a word made from the first letters of other words
acrophobia: fear of heights
acropolis: city built on high ground
acromegaly: abnormal growth of body extremities
4. aer, aero = air
aerate: to let air reach something
aerial: relating to the air
aerospace: the industry of air and space flight
aerobic: requiring free oxygen (as in exercise)
aerosol: particles or droplets carried in air
5. agon = struggle, contest
agony: extreme pain or struggle
protagonist: main (first) character, the leading struggler in a story
antagonist: a person who opposes or struggles against the protagonist
agonize: to suffer extreme pain or struggle mentally/emotionally
agonistic: showing struggle or competition
antagonistic: hostile; actively opposing
antagonize: to cause someone to become hostile
Not a Root-Match (for reference):
hexagon, pentagon, octagon: "gon" means angle (not related to struggle)