Measuring Angles with a ProtractorDate: 02/28/2002 at 07:12:14 From: Rezzi Subject: Angles I want to know how to measure acute, reflex and obtuse angles with a protractor. Date: 02/28/2002 at 09:53:33 From: Doctor Rick Subject: Re: Angles Hi, Rezzi. To measure an acute angle (less than 90 degrees), put the vertex at the center of the straight side of the protractor, lining up one half of that side with one side of the angle, so that the other side is under the protractor. On the curved edge of the protractor are (usually) two sets of numbers; one scale starts with zero on the left edge and increases to the right, while the other starts with zero on the right edge and increases to the left. Choose the scale that starts on the side that you lined up with a side of the angle. Read off the number from this scale at the point where the other side of the angle crosses the protractor. Measuring obtuse angles (between 90 and 180 degrees) works exactly the same way. It may be that some protractors have only one scale, with zero on both ends and 90 in the middle. If so, then you'll read a number between 0 and 90, and you'll need to subtract it from 180 to get the measure of the obtuse angle. For instance, if you read 45, the angle is really 180 - 45 = 135 degrees A reflex angle is the "outside" of an acute or obtuse angle. Use the protractor to read the measure of the "inside" of the angle. Then subtract from 360 to get the measure of the reflex angle. If the inside is the same obtuse angle we just measured, then the measure of the outside is 360 = 135 = 225 degrees Does that help? - Doctor Rick, The Math Forum http://mathforum.org/dr.math/ Date: 07/14/2003 at 16:35:03 From: Gloria Subject: Reading the measurements on the protractor I have different pictures of a protractor with arrows pointing to different numbers on it. None of them starts at 0. This protractor is 0-180. If I have arrows pointing to 80-150, do I start at 0? Or would I just say 70 and subtract from 180? I'm confused! Date: 07/14/2003 at 17:05:39 From: Doctor Peterson Subject: Re: Reading the measurements on the protractor Hi, Gloria. I think you mean that you are supposed to find the angle between the 80 and 150 marks on the protractor. Although you would normally set the protractor so that one leg of the angle lies at zero, you can also use it this way. Remember that a protractor is really just a curved ruler, and can be used in exactly the same ways (apart from having to put the vertex of the angle in the right place). On a ruler, you can measure starting at zero: 0 1 2 3 4 5 6 7 8 +---+---+---+---+---+---+---+---+ ===================== 5 inches or between any two positions: 0 1 2 3 4 5 6 7 8 +---+---+---+---+---+---+---+---+ ===================== 7-2=5 inches You subtract the readings of the two ends to find the distance between them. So the angle between the 80 and 150 marks is just 150-80 = 70 degrees. What you are really doing is subtracting two angles. If the points at the 0, 80, and 150 degree marks are P, Q, and R, and the center of the protractor is at O, then you are measuring angles QOR as POR - POQ. Draw a picture of that and see if it makes sense. 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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