From corresponding points to function notation

Read stretches and translations from labelled graphs, then write the function.

  • Compare horizontal distances between corresponding points to find the horizontal scale factor. Then find the horizontal translation.
  • Use vertical distances and vertical positions in the same way. Check every labelled point against the final function.

Example 1

The intercepts A(−4, 0), B(0, 0) and C(6, 0) become A(−10, 0), B(−2, 0) and C(10, 0). Write the new function, then play to check.

−16−14−12−10−8−6−4−2246810121416−7−6−5−4−3−2−11234567xyA(−4, 0)B(0, 0)C(6, 0)A′(−10, 0)B′(−2, 0)C′(10, 0)Original y=f(x)Final image
  1. 1.Stretch parallel to the -axis by scale factor .

  2. 2.Translate by .

AB doubles from 4 to 8 and BC doubles from 6 to 12. Use a horizontal stretch of 2, then shift 2 left: .

Example 2

The points A(−2, 0), B(0, −4) and C(2, 0) become A(−2, 3), B(0, −5) and C(2, 3). Write the new function, then play to check.

−4−3−2−11234−12−10−8−6−4−224681012xyA(−2, 0)B(0, −4)C(2, 0)A′(−2, 3)B′(0, −5)C′(2, 3)Original y=f(x)Final image
  1. 1.Stretch parallel to the -axis by scale factor .

  2. 2.Translate by .

The vertical gap from B to A doubles from 4 to 8. Original zero outputs finish at 3, so .

Remember
Use distances to identify the stretch, then a point to identify the translation. Checking several points helps rule out a mistaken order.

Your turn

5 practice questions
Question 1

The black graph is . The blue graph is its image. Use the corresponding points to describe the transformations and write the new function.

−14−12−10−8−6−4−22468101214−6−5−4−3−2−1123456xyA(−4, 0)B(0, 0)C(6, 0)A′(−11, 0)B′(−3, 0)C′(9, 0)Original y=f(x)Image
Check answer
Step 1: Stretch parallel to the -axis by scale factor . .Step 2: Translate by . .

Horizontal operations act on the current -coordinates; vertical operations act on the current -coordinates.

Question 2

The black graph is . The blue graph is its image. Use the corresponding points to describe the transformations and write the new function.

−4−3−2−11234−15−10−551015xyA(−2, 0)B(0, −4)C(2, 0)A′(−2, −3)B′(0, −15)C′(2, −3)Original y=f(x)Image
Check answer
Step 1: Stretch parallel to the -axis by scale factor . .Step 2: Translate by . .

Horizontal operations act on the current -coordinates; vertical operations act on the current -coordinates.

Question 3

The black graph is . The blue graph is its image. Use the corresponding points to describe the transformations and write the new function.

−8−7−6−5−4−3−2−112345678−6−5−4−3−2−1123456xyA(−4, 0)B(0, 0)C(6, 0)A′(−4, 0)B′(−2, 0)C′(1, 0)Original y=f(x)Image
Check answer
Step 1: Stretch parallel to the -axis by scale factor . .Step 2: Translate by . .

Horizontal operations act on the current -coordinates; vertical operations act on the current -coordinates.

Question 4

The black graph is . The blue graph is its image. Use the corresponding points to describe the transformations and write the new function.

−4−3−2−11234−8−7−6−5−4−3−2−112345678xyA(−2, 0)B(0, −4)C(2, 0)A′(−2, −2)B′(0, −10/3)C′(2, −2)Original y=f(x)Image
Check answer
Step 1: Stretch parallel to the -axis by scale factor . .Step 2: Translate by . .

Horizontal operations act on the current -coordinates; vertical operations act on the current -coordinates.

Question 5

The black graph is . The blue graph is its image. Use the corresponding points to describe the transformations and write the new function.

−26−24−22−20−18−16−14−12−10−8−6−4−22468101214161820222426−6−5−4−3−2−1123456xyA(−4, 0)B(0, 0)C(6, 0)A′(19, 0)B′(3, 0)C′(−21, 0)Original y=f(x)Image
Check answer
Step 1: Stretch parallel to the -axis by scale factor . .Step 2: Reflect in the -axis. .Step 3: Translate by . .

Horizontal operations act on the current -coordinates; vertical operations act on the current -coordinates.