Angles as Turns
Date: 05/29/2003 at 10:05:27 From: Jimmy Subject: Angles How can angles be negative? I don't know what negative angles are.
Date: 05/29/2003 at 12:46:36 From: Doctor Peterson Subject: Re: Angles Hi, Jimmy. This is similar to asking how a distance can be negative. Normally we think of distances as lengths, which must be positive. But sometimes we are thinking of directed distances, such as how far we are above sea level; then a distance below sea level would be "backward," and would be given a negative sign. Similarly, in geometry we normally think of an angle as just the "distance" between two rays, and it is always positive. But (particularly in trigonometry) we can also think of an angle as a rotation, which can go either clockwise or counterclockwise, so the direction counts. Just as we think of "up" as positive, someone decided long ago to think of a counterclockwise turn as positive (probably because an angle in standard position, with the starting ray along the x axis, starts out going up when you go counterclockwise). So a counterclockwise angle is positive, and a clockwise angle is negative: / / up / ^ / | /+ | clockwise +---------- +---------- counterclockwise \- | \ | \ v \ down \ When the angle is not in standard position, we still think of a clockwise turn as negative. So here's the main point: when we think of an angle just as a figure on the paper, and want to measure how big it is, the answer is always positive. But when we think of an angle as a turn, starting at one ray and ending at another, then we will include a sign in the answer, showing which direction it turns. If you have any further questions, feel free to write back. - Doctor Peterson, The Math Forum http://mathforum.org/dr.math/
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