When they are equal in magnitude but opposite in direction.
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When the angle between two vectors is zero ... i.e. the vectors are parallel ... their sum is a vector in thesame direction, and with magnitude equal to the sum of the magnitudes of the two original vectors.
Two is the minimum number of vectors that will sum to zero.
They need equal magnitudes and opposite directions.
In order for two vectors to add up to zero:-- their directions must be exactly opposite-- their magnitudes must be exactly equal
Their sum can be zero only if their magnitudes are equal and their directions are exactly opposite.