8. A horizontal line (y=3) is perpendicular to a vertical line (x=0).
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9. A horizontal line (y=2) is parallel to a horizontal line (y=-1/2). The lines are not perpendicular.
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10. A parallelogram is a rectangle if the sides are perpendicular. The slopes of the short sides are rise/run = -1, and the slopes of the long sides are rise/run = +1. These values have a product of -1, so the sides are perpendicular. The figure is a rectangle.
