Let X1, X2, and x3 be vectors in R3. If x1 is perpendicular to X2 and X2 is perpendicular to x3, is it necessarily true that x1 is perpendicular to x3? Prove your
answer.

Atrain leaves little? rock, arkansas, and travels north at 90 kilometers per hour. another train leaves at the same time and travels south at 80 kilometers per hour. how long will it take before they are 680 kilometers? apart?