A right angle is an angle that measures exactly 90 degrees, a quarter of a full turn. It looks like the corner of a square or a sheet of paper, and it is marked with a small square at the vertex. Two lines that meet at a right angle are perpendicular.
The small square is the signal for 90°. Compare: an acute angle is narrower, an obtuse angle is wider.
Formal definition and notation
A right angle is an angle whose measure is 90°, or π2radians: one fourth of a full rotation of 360°. Two rays, segments or lines that form a right angle are perpendicular, written ⊥. An angle smaller than a right angle is acute, and one between 90° and 180° is obtuse.
A right angle is one 90° angle. A right triangle is a triangle that contains one right angle (its other two angles are acute).
Right Anglevs.Straight angle
A straight angle is 180°, a flat line, which is two right angles side by side.
Quick check: 3 questions
Question 1 of 3
Which angle measure is a right angle?
Show answer and explanation
Answer: C. A right angle is exactly 90°. 45° is acute, 100° is obtuse, and 180° is a straight angle.
Question 2 of 3
Two perpendicular lines cross. How many right angles do they form?
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Answer: C. Perpendicular lines make four angles where they cross, and all four are right angles. Choice A counts only the marked corner.
Question 3 of 3
An angle measures 92°. What kind of angle is it?
Show answer and explanation
Answer: A. Any angle between 90° and 180° is obtuse. A right angle must be exactly 90°, so 92° does not count even though it looks almost square.
Right Angle: frequently asked questions
Can a triangle have two right angles?
No. The three angles of a triangle add to 180 degrees. Two right angles would already use 90 + 90 = 180 degrees, leaving 0 degrees for the third angle, so the two sides would run parallel and never meet. A triangle has at most one right angle, while a rectangle or a square has four.
How can you check for a right angle without a protractor?
Use the corner of a sheet of paper or an index card, which is a right angle. Put the corner on the vertex and line up one edge with one side of the angle. If the other side lines up with the other edge, it is a right angle; if it falls inside the paper, the angle is acute, and outside means obtuse.
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