Acute and obtuse angles

Mathematics · Geometry · ages 8-9

Name ______________________ Date ____________

Which of these could be an acute angle?
- One that is exactly a square corner
- One that opens wider than a square corner
- One that opens smaller than a square corner
What kind of angle is the corner of a rectangular book cover?
- Acute
- Right
- Obtuse
A drawing shows four openings. One opens smaller than a right angle, one matches a right angle exactly, one is wider than a right angle but less than a half turn, and one is a half turn. How many of the openings are acute?
Answer: ______________
An angle matches a square corner exactly. What type of angle is it?
- Acute
- Right
- Obtuse
- Straight
A pentagon has 5 corners. One corner is a right angle, two corners are acute, and the rest are obtuse. How many obtuse angles does the pentagon have?
Answer: ______________
Mia draws an angle that opens a little, less than a right angle. What type of angle did she draw?
- Acute
- Right
- Obtuse
Pia draws a right angle. She wants to turn it into an obtuse angle, keeping the final opening less than a half turn. What should she do?
- Close it smaller
- Make the lines longer
- Open it wider, but less than a half turn
Which list has only times where the smaller opening between the clock hands is an acute angle?
- 1 o'clock and 2 o'clock
- 1 o'clock and 4 o'clock
- 2 o'clock and 5 o'clock
At 7 o'clock, the minute hand points straight up to 12 and the hour hand points to 7. What kind of angle do the hands make at the smaller opening?
- Acute
- Right
- Obtuse
Four angles are in order from smallest to largest: acute, right, obtuse, straight. How many of them are less than a half turn?
Answer: ______________

Answer key

For grown-ups. Fold this page away before handing over the rest.

Acute and obtuse angles W1-mt_h0CVtqI2xo-s1

One that opens smaller than a square corner · Acute angles are always smaller than a right angle, so an acute angle opens smaller than a square corner.
Right · A rectangular book cover corner is a square corner, and a square corner is a right angle.
1 · Only the opening smaller than a right angle is acute. The others are right, obtuse, and straight, so the count is 1.
Right · An angle that matches a square corner exactly is a right angle.
2 · It has 2 obtuse angles because 5 corners minus 1 right and 2 acute leaves 2.
Acute · An angle that opens a little, less than a right angle, is acute. So Mia drew an acute angle.
Open it wider, but less than a half turn · Obtuse means wider than a right angle but less than a half turn. Opening wider, but less than a half turn, makes it obtuse.
1 o'clock and 2 o'clock · At 1 o'clock and 2 o'clock the hands sit close together, so both smaller openings are acute. The other lists mix in a wider, obtuse time.
Obtuse · At 7 o'clock the smaller opening is more than a right angle but less than a half turn, so the angle is obtuse.
3 · Acute, right, and obtuse are all less than a half turn. Straight equals a half turn, so it is not less. That makes 3.