 ## Grade 4 Math: Geometry: Angles and Lines, Part 1

### Bilateral Symmetry

Bilateral symmetry is defined as the fact that two halves of an object or figure are identical. Any object or figure for which a line can divide the object or figure into two parts and the parts are identical, the object or figure has bilateral symmetry.

### Angles

In an earlier lesson, students have learned that an angle is defined by the rotation of a ray around a point, and that the amount of rotation is measured in degrees.

In this lesson, they learn about different types of angles defined by the amount of rotation. A full angle is defined as a full rotation, which is 360 degrees. A straight angle is defined as half of a full rotation, or 180 degrees. A right angle is defined as one-fourth of a full rotation, or 90 degrees.

Two other angles are defined relative to a right angle. An acute angle is an angle that is more than zero degrees and less than 90 degrees. An obtuse angle is an angle that is more than 90 degrees but less than 180 degrees.

Students learn to identify these angles visually.

### Comparing Angles

In this lesson, students learn to compare the size of two angles. If two angles have the same number of degrees, they are the same size. If a given angle has more degrees than a second angle, it is larger than that angle. If a given angle has fewer degrees than a second angle, it is smaller than that angle.

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