When 0.4i+0.8j+ck will represent a unit vector if ,
magnitude of the vector=1 => square root{(0.4)^2+(0.8)^2+c^2}=1
=> 0.16+0.64+c^2=1=> c= square root (0.2)
I hope my answer helps. All the very best for your future endeavors!
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