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What is the geometric effect of scalar multiplication when the scalar is negative?

Multiplying by a negative number flips the vector's direction while adjusting its length based on the absolute value of the scalar.

Conditions

  • Scalar c<0c < 0
  • Non-zero initial vector

Reasoning, step by step

  1. Calculate the new length as original length times ∣c∣|c|.
  2. Reverse the direction of the vector by 180 degrees.
  3. Combine these effects to form the scaled vector.

Example

Multiplying [20]\begin{bmatrix} 2 \\ 0 \end{bmatrix} by −1-1 results in [−20]\begin{bmatrix} -2 \\ 0 \end{bmatrix}, which points left instead of right.

Common misconceptions

  • Thinking the vector shrinks to zero if the scalar is negative.
  • Confusing sign change with rotation other than 180 degrees.

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