Step by step
The question
Question: find the residual for the line of best fit y = 2.5x and the point (2, 7).
The point is the real data.
y = 2.5x; (2, 7)
A residual tells how far a point is from the line.
Residual = actual minus predicted.
Actual is the real y.
Predicted is the y the line gives.
residual = actual − predicted
1 Predict
We put x = 2 into the line.
It predicts y = 5.
predicted = 2.5(2) = 5
2 Actual minus predicted
Careful!
Not predicted minus actual.
That flips the sign.
Actual comes first.
Careful: not 5 − 7 = −2 ✗
The actual y is 7.
7 minus 5 is 2.
residual = 7 − 5 = 2
So the residual is 2.
It is positive: the point is 2 above the line.
3 Check
Let's check with a picture.
The line of best fit.
The point.
The residual is the distance up or down from the line to the point.
And predicted plus residual gives the actual y: 7.
So the answer is right!
5 + 2 = 7
Let's look back.
First: predict.
Then: actual minus predicted.
Last: check.
Answer
residual = 7 − 5 = 2