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In this lesson, we will learn

- what is rotation
- how to draw the rotated image of an object given the center, the angle and the direction of rotation.
- how to find the angle of rotation given the object, its image and the center of rotation.
- how to rotate points and shapes on the coordinate plane about the origin.

More Geometry Lessons

A **rotation** is a
transformation in which the object is rotated about a fixed point.
The **direction of rotation** can
be **clockwise** or **anticlockwise**.

The fixed point in which the rotation takes place is called the
**centre of rotation**. The amount
of rotation made is called the **angle
of rotation**.

* Example: *

For any rotation, we need to specify the center, the angle and
the direction of rotation.

Given the center of rotation and the angle of rotation we can determine the rotated image of an object.

*Example: *

Determine the image of the straight line *XY* under an
anticlockwise rotation of 90˚ about *O*.

* Solution: *

**Step 1: **Join
point *X* to *O. *

** Step 2: ** Using a
protractor, draw a line 90˚ anticlockwise from the line *OX. *Mark
on the line the point *X*’ such that the line of *OX*
= *OX*’

**Step 3: ** Repeat
steps 1 and 2 for point *Y*. Join the points *X’*
and *Y*’ to form the line *X’Y’. *

Given an object, its image and the centre of rotation, we can find the angle of rotation using the following steps.

**Step 1 : **Choose
any point in the given figure and join the chosen point to the
centre of rotation.

**Step 2** : Find
the image of the chosen point and join it to the centre of
rotation.

** Step 3** : Measure the angle between
the two lines. The sign of the angle depends on the direction of
rotation. Anti-clockwise rotation is positive and clockwise
rotation is negative.

* Example*

Figure *A*’*B*’*C*’ is the image of figure
*ABC. O* is the centre of rotation. Find the angle of
rotation.

* Solution*

** Step 1: ** Join *A*
to *O*

** Step 2: ** Join *A’*
to *O*.

** Step 3: ** Measure
the angle *AOA’ *

The angle of rotation is 62˚ anticlockwise or +62˚

How to rotate a figure around a fixed point using a compass and protractor.

We will now look at how points and shapes are rotated on the coordinate plane. It will be helpful to note the patterns of the coordinates when the points are rotated about the origin at different angles.

Geometry Rotation

A rotation is an isometric transformation: the original figure and
the image are congruent. The orientation of the image also stays
the same, unlike reflections. To perform a geometry rotation, we
first need to know the point of rotation, the angle of rotation,
and a direction (either clockwise or counterclockwise). A rotation
is also the same as a composition of reflections over intersecting
lines.

The following videos show clockwise and anticlockwise rotation of 0˚, 90˚, 180˚ and 270˚about the origin (0, 0). The pattern of the coordinates are also explored.

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