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Opposite numbers are any two numbers whose sum is zero (–2 and +2 are opposite numbers).
Parallel refers to lines that never intersect.
Percent is a way of expressing a number as a fraction of 100. (Percent means “per hundred.”) The percent symbol is %. So 1/2 expressed as a percentage is 50/100, which is 50%.

The perimeter is the distance around the edge of a multi-sided figure. If a triangle has three sides, each 3 feet long, its perimeter is 9 feet (3 + 3 + 3 = 9).
Perpendicular refers to two lines that intersect, forming right angles (90° angles).