Measure angles in whole-number degrees using a protractor; draw given angles and sketch angles of specified measure
Sometimes you need an angle's exact size. A standard semicircular protractor is a curved tool marked with two rows of numbers from 0 to 180. It measures a selected angle up to 180 degrees. The curved scale sits over the opening. Place its center point on the vertex. Align the printed baseline and zero with one ray. The printed baseline may sit above the physical bottom edge. Read where the other ray crosses, following the same row that starts at that zero.
Look at an angle where the protractor center is on the vertex, one ray lines up with 0, and the other ray crosses at the 47 mark. Follow the same row that starts at 0 on your aligned ray. Read 47. The angle measures 47 degrees. This is acute, a positive opening smaller than a right angle.
A standard semicircular protractor has two rows of numbers, both running from 0 to 180. Each mark has two numbers, one on each row. At 90 both rows agree. Follow the row whose 0 sits on your aligned starting ray. Use the opening as a check: an acute angle, greater than 0 and under 90, reads under 90. An obtuse angle, greater than 90 and under 180, reads between 90 and 180.
To draw a 135 degree angle, first draw a straight ray. Place the protractor center on the ray's end. Align the printed baseline and 0 with the ray. Find 135 on the same row that starts at 0. Mark a point there. Draw the second ray from the vertex through that point.
The two rows count in opposite directions from opposite baseline ends, so they add to 180 at every mark. To find the other row's number, subtract from 180. For example, 180 minus 152 is 28. Count up from 152 to 160 is 8, and 160 to 180 is 20, so 8 plus 20 is 28.
Center on the vertex, align one ray with 0, then read the same row where the other ray crosses. Both rows run 0 to 180. Follow the row whose zero is on your aligned ray.
Ten fresh original tasks (x1-x10) authored for this topic. Three Khan Academy articles retained and adapted: Measuring angles review (academy:article:4th-engage-ny:x6f8c24d9), Constructing angles review (academy:article:4th-engage-ny:x8f475dd2), and Angle basics review (academy:article:4th-engage-ny:x961121b324233fec). Two Khan Academy videos retained: Measuring angles using a protractor (academy:video:4th-engage-ny:263439062) and Measuring angles with a circular protractor (academy:video:4th-engage-ny:xc20e9a03). All corpus licenses are null and unverified.
Priya has a kite plan with one corner that has no number on it. She wants its exact size. She uses a standard semicircular protractor, a curved tool with two rows of numbers from 0 to 180. The first move is placing the center point on the vertex, the corner where the two rays meet.
Where does the center point of the protractor go?
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