Step by step
The question
Question: rotate the point P(0, 3) 180° about the point (2, 1).
1 Draw the shape
Put the point on the grid.
The center of the turn is (2, 1).
Everything turns about this point.
2 Turn one point
From the center to P: go left 2
then up 2.
Now turn 180° about the center.
The path from the center turns too.
Left 2 turns into right 2.
Up 2 turns into down 2.
We land on P', (4, −1).
We call the new point P prime.
The little mark means: the new P.
3 Find the rule
Now the same with numbers, for P.
The turn rules we know turn about the origin.
So first we move the center to the origin: take (2, 1) away from P.
P is (−2, 2) from the center.
Turn 180°: the rule (x, y) goes to (−x, −y).
(−2, 2) becomes (2, −2).
Then move back: add (2, 1).
We get (4, −1): P', the same as on the grid!
Careful!
That is a turn about the origin.
The center here is (2, 1), so we take it away first and add it back at the end.
So the image is P'(4, −1).
4 Check
Let's check P.
Join the center to P
and the center to P'.
From the center, P is (−2, 2) away and P' is (2, −2) away.
Both have length √8: the same distance, as a turn must keep.
And P, the center and P' are on one straight line: a half turn.
Right!
Let's look back.
First: draw the shape.
Then: turn one point.
Then: find the rule.
Last: check.