Classify Angles: Acute, Right or Obtuse
Skill: Identifying acute, right and obtuse angles
Download PDF worksheetAnswer key (PDF)
Free for personal and classroom use. Print at 100% / "Actual size" on US Letter paper.
About this worksheet
Angles are everywhere in geometry, from the corners of a book to the hands of a clock. This fourth-grade worksheet shows twelve angles drawn in different positions. Students decide whether each angle is acute, right, or obtuse and write the word on the line. Right angles are marked with the small square symbol used in math books, while other angles are marked with an arc.
The angles are rotated in different directions on purpose. Students often recognize a right angle only when one side is flat along the bottom, so turning the angles helps them focus on the opening between the rays instead of the position on the page. The acute and obtuse angles are clearly different from 90 degrees, so each answer can be found by careful looking.
How to use it: give students a simple tool, the corner of an index card or a folded piece of paper, which makes a perfect right angle. They can hold the corner up to each angle. If the angle opens less than the corner, it is acute. If it matches exactly, it is right. If it opens wider, it is obtuse.
The answer key gives the correct classification for every angle. This worksheet supports Common Core standards 4.G.A.1 (draw and identify points, lines, line segments, rays, angles of right, acute, and obtuse types, and parallel and perpendicular lines) and 4.MD.C.5 (recognize angles as geometric shapes formed by two rays sharing a common endpoint).
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