Perpendicular Vectors

Question

Determine a value of t so that vector u = [9,t] and vector v = [-16,t] are perpendicular.

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Also, are projections of perpendicular vectors always 0? A friend stated that they were but that doesn't quite make sense to me.

Answer 1

Dot product u·v = 0; 9·(-16) + t·t = 0; t² = 144; t = ±12

Answer 2

Two vectors are perpendicular the dot product of the two is 0, zero.

(9)(-16) + (t)(t) = 0.

Draw two perpendicular vectors on a piece of paper. Project them on another piece of paper. When in the projection equal to 0?