Step by step
The question
Question: rotate the shape A(2, 1), B(4, 1) and C(4, 3) 90° clockwise about the origin.
1 Draw the shape
Put the points on the grid and join them.
2 Turn one point
Clockwise means the way the hands of a clock turn.
Start at the center O.
To reach A: go right 2
then up 1.
Now turn a quarter turn clockwise about O.
Everything turns with it.
Right 2 turns into down 2.
Up 1 turns into right 1.
We land on the new point A'.
We call the new point A prime.
The little mark means: the new A.
3 Find the rule
Look at the numbers of A and A'.
A(2, 1) goes to A'(1, −2).
Careful!
A quarter turn swaps the two numbers.
Changing a sign alone is a flip, not a turn.
The two numbers swap places, and the new second number gets a minus sign.
So the rule is: (x, y) goes to (y, −x).
4 Turn the other points
B is (4, 1): swap and change the sign of the second: B' is (1, −4).
C is (4, 3): swap and change the sign of the second: C' is (3, −4).
Join the new points.
This new shape is called the image.
So the image is A'(1, −2), B'(1, −4) and C'(3, −4).
5 Check
Let's check B on the grid.
Join O to B
and O to B'.
They make a right angle: a quarter turn.
And it turns clockwise.
Right!
Let's look back.
First: draw the shape.
Then: turn one point.
Then: find the rule.
Then: turn the other points.
Last: check.