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Precalculus
Functions and their graphs
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1. Function notation: evaluate and read f(x)
Discover
f(x)=x+5 is a rule: it takes a number and adds 5.
What does f(2) mean?
The output of f when the input is 2
f times 2
The input that gives the output 2
Key idea
A function gives exactly one output for each input. We write the output as f(x).
f(a) is the output of f for the input a. To find it, replace every x in the rule by (a), with parentheses.
On a graph, f(a) is the height of the graph above x=a.
Worked example
f(x)=3x2−x. Find f(−2).
f(−2)=3(−2)2−(−2)
Replace each x by (−2), with the parentheses.
=3⋅4+2
(−2)² = 4, and subtracting −2 adds 2.
=14
Multiply, then add.
Your turn, guided
Your turn: f(x)=2x2+1. Fill in the missing line.
f(−3)=2(−3)2+1
Replace x by (−3).
f(−3)=19
(−3)² = 9, then 2 · 9 + 1.
Practice ★
f(x)=2x2+3. Find f(−3).
f(x)=4x2+8. Find f(−2).
f(x)=4x2+2. Find f(−5).
f(x)=5x2+5. Find f(−4).
f(x)=3x2+3. Find f(−4).
Watch out!
Practice ★★
f(x)=x2+3. Find f(2+3).
f(x)=x2+2. Find f(4+4).
f(x)=x2+7. Find f(4+1).
f(x)=x2+3. Find f(3+3).
Here is the graph of y=f(x). Tap the point of the graph that shows f(2).
Here is the graph of y=f(x). Tap the point of the graph that shows f(2).
Exit ticket
Three questions to check you have got it.
f(x)=x2+3. Find f(2+3).
f(x)=x2+2. Find f(4+4).
f(x)=x2+7. Find f(4+1).
Summary card
Function notation
f(a) is the output for the input a.
Replace every x by (a), with parentheses: f(−2) for x2 is (−2)2=4.
On a graph: the height above x=a.
Watch out: f(a+b) is not f(a)+f(b).
2. Domain and range in interval notation
Warm-up
Three quick questions from earlier lessons.
Discover
Which numbers can you put into f(x)=x?
0 and the positive numbers
Every number
Only the positive numbers, not 0
Key idea
The domain is the set of inputs a function accepts. The range is the set of outputs it gives.
Leave out of the domain every x that makes a denominator 0 or puts a negative number under a square root. A square bracket includes the end: [2,∞) contains 2; a parenthesis leaves it out: (2,∞) does not.
Worked example
Find the domain of f(x)=x+2.
x+2≥0
The inside of a square root must not be negative.
x≥−2
Subtract 2 from both sides.
Domain: [−2,∞)
−2 is allowed (√0 = 0), so use a square bracket.
Your turn, guided
Your turn: f(x)=x−5. Fill in the missing line.
x−5≥0
The inside must not be negative.
The domain starts at x=5: [5,∞)
Add 5 to both sides: the domain starts at this number.
Practice ★
What is the domain of f(x)=x−9? Give it as an interval.
What is the domain of f(x)=x−5? Give it as an interval.
What is the domain of f(x)=x−6? Give it as an interval.
What is the domain of f(x)=x−4? Give it as an interval.
Watch out!
Practice ★★
What is the domain of f(x)=x−69? Give it as an interval or a union of intervals.
What is the domain of f(x)=x−56? Give it as an interval or a union of intervals.
What is the domain of f(x)=x−34? Give it as an interval or a union of intervals.
What is the range of f(x)=(x−5)2+9? Give it as an interval.
What is the range of f(x)=(x−3)2+6? Give it as an interval.
What is the range of f(x)=(x−5)2+4? Give it as an interval.
Exit ticket
Three questions to check you have got it.
What is the range of f(x)=(x−5)2+9? Give it as an interval.
What is the range of f(x)=(x−3)2+6? Give it as an interval.
What is the range of f(x)=(x−5)2+4? Give it as an interval.
Summary card
Domain and range
Domain: the inputs. Leave out zero denominators and negatives under a square root.
Range: the outputs.
[a,∞) includes a; (a,∞) does not.
Watch out: the range uses y-values, not the x of a vertex.
3. Piecewise-defined functions and absolute value
Warm-up
Three quick questions from earlier lessons.
Discover
A parking lot charges 5 dollars for up to 2 hours, and 9 dollars for a longer stay. What does a stay of exactly 2 hours cost?
5 dollars
9 dollars
Key idea
A piecewise function uses a different formula on different parts of its domain.
To find f(a): first find which condition a satisfies, then use that piece's formula. Check the boundary carefully (< or ≤).
The absolute value is piecewise: ∣x∣=x if x≥0, and ∣x∣=−x if x<0.
Worked example
f(x)={x22x+3if x<1if x≥1. Find f(1).
Use the second piece, 2x+3
Check the conditions first: 1 is not less than 1, but 1 ≥ 1.
f(1)=2⋅1+3=5
Replace x by 1.
Your turn, guided
Your turn, with the same f: find f(−2).
Use the first piece, x2
−2 < 1.
f(−2)=4
(−2)² = 4.
Practice ★
f(x)={x+44xif x<8if x≥8. Find f(0).
f(x)={x+84xif x<7if x≥7. Find f(2).
f(x)={x+73xif x<3if x≥3. Find f(1).
f(x)={x+64xif x<6if x≥6. Find f(1).
Watch out!
Practice ★★
f(x)={x+33xif x<5if x≥5. Find f(5).
f(x)={x+42xif x<3if x≥3. Find f(3).
f(x)={x+15xif x<1if x≥1. Find f(1).
f(x)={x+22xif x<5if x≥5. Find f(5).
For x<7, which expression is equal to ∣x−7∣?
7−x
x−7
−x−7
For x<5, which expression is equal to ∣x−5∣?
5−x
x−5
−x−5
Exit ticket
Three questions to check you have got it.
f(x)={x+33xif x<5if x≥5. Find f(5).
f(x)={x+42xif x<3if x≥3. Find f(3).
f(x)={x+15xif x<1if x≥1. Find f(1).
Summary card
Piecewise functions
Find the piece whose condition the input satisfies.
Use that piece's formula.
∣x∣=x for x≥0, and −x for x<0.
Watch out: at a boundary, read < and ≥ carefully.
4. Average rate of change; where a function increases or decreases
Warm-up
Three quick questions from earlier lessons.
Discover
A car drives 120 miles in 2 hours. What is its average speed?
60 miles per hour
601 hour per mile
120 miles per hour
Key idea
The average rate of change tells how fast the output changes, per unit of input.
The average rate of change of f from x=a to x=b is b−af(b)−f(a), the slope of the line through the two points of the graph.
f is increasing on an interval if its graph goes up from left to right there, and decreasing if it goes down.
Worked example
Find the average rate of change of f(x)=x2 from x=1 to x=4.
f(4)−f(1)=16−1=15
Change in the output.
4−1=3
Change in the input.
Rate =315=5
Divide: output change per unit of input.
Your turn, guided
Your turn: f(x)=2x2, from x=0 to x=3.
f(3)−f(0)=18
f(3) = 18, f(0) = 0.
Change in x: 3
3 − 0.
Rate =6
18 ÷ 3.
Practice ★
Find the average rate of change of f(x)=3x2 from x=0 to x=6.
Find the average rate of change of f(x)=4x2 from x=1 to x=5.
Find the average rate of change of f(x)=1x2 from x=1 to x=6.
Find the average rate of change of f(x)=1x2 from x=1 to x=4.
Watch out!
Practice ★★
A tank holds 16 liters at time 0 and 61 liters after 5 minutes. What is the average rate of change of the volume, in liters per minute?
A tank holds 29 liters at time 0 and 49 liters after 4 minutes. What is the average rate of change of the volume, in liters per minute?
A tank holds 12 liters at time 0 and 36 liters after 6 minutes. What is the average rate of change of the volume, in liters per minute?
f(x)=−(x−5)2+8. On which interval is f increasing?
(−∞,5)
(5,∞)
(−∞,8)
f(x)=−(x−1)2+7. On which interval is f increasing?
(−∞,1)
(1,∞)
(−∞,7)
f(x)=−(x−3)2+2. On which interval is f increasing?
(−∞,3)
(3,∞)
(−∞,2)
Exit ticket
Three questions to check you have got it.
Find the average rate of change of f(x)=3x2 from x=0 to x=6.
Find the average rate of change of f(x)=4x2 from x=1 to x=5.
Find the average rate of change of f(x)=1x2 from x=1 to x=6.
Summary card
Average rate of change
From a to b: b−af(b)−f(a), output change over input change.
Increasing: the graph rises from left to right.
Give intervals of increase with x-values.
Watch out: change in f(x) on top, change in x below.
5. Composition of functions
Warm-up
Three quick questions from earlier lessons.
Discover
g doubles a number: g(x)=2x. f adds 1: f(x)=x+1. Start with 3: double it, then add 1. You have computed f(g(3)). What is it?
7
8
24
Key idea
Composing means using the output of one function as the input of another.
f(g(x)): first apply g to x, then apply f to the result. Work from the inside out. In general f(g(x))=g(f(x)).
Worked example
f(x)=x2 and g(x)=x−3. Find f(g(5)), then f(g(x)).
g(5)=5−3=2
The inside first.
f(g(5))=f(2)=4
Then f of that result.
f(g(x))=(x−3)2
With x: put g(x) = x − 3 in place of x in f.
Your turn, guided
Your turn: f(x)=2x+1 and g(x)=x2. Find f(g(3)).
g(3)=9
The inside first.
f(g(3))=f(9)=19
Then 2 · 9 + 1.
Practice ★
f(x)=2x+2 and g(x)=x2−2. Find f(g(3)).
f(x)=3x+3 and g(x)=x2−4. Find f(g(3)).
f(x)=4x+3 and g(x)=x2−1. Find f(g(2)).
f(x)=3x+1 and g(x)=x2−5. Find f(g(3)).
Watch out!
Practice ★★
f(x)=6x+6 and g(x)=x+5. Write f(g(x)).
f(x)=5x+7 and g(x)=x+2. Write f(g(x)).
f(x)=6x+8 and g(x)=x+3. Write f(g(x)).
g(1)=5, g(5)=7, f(1)=7 and f(5)=1. Find f(g(1)).
g(2)=6, g(6)=7, f(2)=7 and f(6)=2. Find f(g(2)).
Exit ticket
Three questions to check you have got it.
f(x)=2x+2 and g(x)=x2−2. Find f(g(3)).
f(x)=3x+3 and g(x)=x2−4. Find f(g(3)).
f(x)=4x+3 and g(x)=x2−1. Find f(g(2)).
Summary card
Composition
f(g(x)): apply g first, then f.
Work from the inside out.
The order matters: f(g(x)) and g(f(x)) usually differ.
Watch out: f(g(x)) is not f(x)⋅g(x).
6. Transformations: shifts and reflections
Warm-up
Three quick questions from earlier lessons.
Discover
Move the sliders and watch the graph of y=(x−h)2+k.
The graph of y = (x − h)² + k, a parabola whose vertex is at (h, k) and moves with the sliders.
When h goes from 0 to 2, which way does the graph move?
To the right
To the left
Key idea
Changing the rule moves or flips the graph without changing its shape.
y=f(x−h)+k moves the graph of f by h units to the right and k units up. y=−f(x) reflects it across the x-axis; y=f(−x) reflects it across the y-axis.
A change inside f(…) acts on x (horizontally); a change outside acts on y (vertically).
Worked example
Describe y=(x+1)2−3 compared with y=x2.
1 unit to the left
x + 1 = x − (−1): h = −1.
3 units down
− 3 outside: k = −3.
Vertex: (−1,−3)
The vertex (0, 0) moves with the graph.
Your turn, guided
Your turn: y=(x−4)2+2.
4 units to the right
x − 4: h = 4.
2 units up
+ 2 outside.
The vertex is at x=4
The vertex moves from x = 0 to x = …
Practice ★
The graph of y=x2 is moved 1 units to the right and 6 units down. Write the equation of the new graph.
The graph of y=x2 is moved 3 units to the right and 1 units down. Write the equation of the new graph.
The graph of y=x2 is moved 4 units to the right and 6 units down. Write the equation of the new graph.
The graph of y=x2 is moved 7 units to the right and 1 units down. Write the equation of the new graph.
Watch out!
Practice ★★
Draw the graph of y=(x−1)2−3: place the vertex and one more point.
Draw the graph of y=(x−1)2−1: place the vertex and one more point.
Match each equation with what it does to the graph of y=f(x).
y=f(x)+3
y=f(x−3)
y=−f(x)
y=f(−x)
moves it up 3 units
moves it 3 units to the right
reflects it across the x-axis
reflects it across the y-axis
Exit ticket
Three questions to check you have got it.
The graph of y=x2 is moved 1 units to the right and 6 units down. Write the equation of the new graph.
The graph of y=x2 is moved 3 units to the right and 1 units down. Write the equation of the new graph.
The graph of y=x2 is moved 4 units to the right and 6 units down. Write the equation of the new graph.
Summary card
Shifts and reflections
f(x−h): h units right. f(x)+k: k units up.
−f(x): flip across the x-axis. f(−x): flip across the y-axis.
Inside f(…) acts on x; outside acts on y.
Watch out: f(x+3) moves the graph left.
7. Stretches and compressions; even and odd functions
Warm-up
Three quick questions from earlier lessons.
Discover
Move the slider and watch the graph of y=(bx)2.
The graph of y = (bx)², a parabola that gets narrower as b grows.
When b goes from 1 to 2, the graph becomes…
narrower
wider
Key idea
Multiplying stretches or squeezes a graph. Symmetry has a name.
For a>1: y=af(x) stretches the graph vertically by a. For b>1: y=f(bx) compresses it horizontally by b. f is even if f(−x)=f(x) (symmetric about the y-axis), odd if f(−x)=−f(x) (symmetric about the origin).
Worked example
Is f(x)=x3−2x even, odd, or neither?
f(−x)=(−x)3−2(−x)
Replace x by (−x).
=−x3+2x
(−x)³ = −x³ and −2(−x) = 2x.
=−(x3−2x)=−f(x): odd
Factor out −1: this is −f(x).
Your turn, guided
Your turn: f(x)=x4+3. Check one value.
f(−1)=(−1)4+3
Replace x by −1.
f(−1)=4
(−1)⁴ = 1. It equals f(1): a sign that f is even.
Practice ★
How does the graph of y=f(4x) compare with the graph of y=f(x)?
It is squeezed toward the y-axis: horizontal compression by a factor of 4
It is pulled away from the y-axis: horizontal stretch by a factor of 4
It is stretched vertically by a factor of 4
How does the graph of y=f(5x) compare with the graph of y=f(x)?
It is squeezed toward the y-axis: horizontal compression by a factor of 5
It is pulled away from the y-axis: horizontal stretch by a factor of 5
It is stretched vertically by a factor of 5
How does the graph of y=f(3x) compare with the graph of y=f(x)?
It is squeezed toward the y-axis: horizontal compression by a factor of 3
It is pulled away from the y-axis: horizontal stretch by a factor of 3
It is stretched vertically by a factor of 3
Watch out!
Practice ★★
f(x)=8x3−9x. Is f even, odd, or neither?
Even
Odd
Neither
f(x)=9x3−2x. Is f even, odd, or neither?
Even
Odd
Neither
f(x)=3x4−8x2+7. Is f even, odd, or neither?
Even
Odd
Neither
f(x)=5x4−1x2+9. Is f even, odd, or neither?
Even
Odd
Neither
Exit ticket
Three questions to check you have got it.
f(x)=8x3−9x. Is f even, odd, or neither?
Even
Odd
Neither
f(x)=9x3−2x. Is f even, odd, or neither?
Even
Odd
Neither
f(x)=7x3−1x. Is f even, odd, or neither?
Even
Odd
Neither
Summary card
Stretches, even and odd
af(x): vertical stretch by a. f(bx): horizontal compression by b.
Even: f(−x)=f(x), mirror image in the y-axis.
Odd: f(−x)=−f(x), symmetric about the origin.
Watch out: compute f(−x); the signs of the coefficients do not decide.
8. Inverse functions
Warm-up
Three quick questions from earlier lessons.
Discover
f(x)=x+7 adds 7. Which function undoes it, and gives back the number you started with?
x−7
x+71
x+7
Key idea
The inverse function f−1 undoes f: it swaps inputs and outputs.
If f(a)=b, then f−1(b)=a. To find f−1(x): write y=f(x), solve for x, then rename. The graph of f−1 is the reflection of the graph of f across the line y=x.
f−1(x) is not f(x)1: the −1 is not an exponent here.
Worked example
f(x)=3x−5. Find f−1(x).
y=3x−5
Name the output y.
y+5=3x
f subtracts 5 last, so undo that first: add 5.
x=3y+5
Then undo × 3: divide by 3.
f−1(x)=3x+5
Rename the input x.
Your turn, guided
Your turn: f(x)=2x+6.
y=2x+6
Name the output y.
y−6=2x
Undo + 6 first.
f−1(x)=2x−6
Divide by 2, then rename.
Practice ★
f(x)=7x+9. Find f−1(x).
f(x)=2x+2. Find f−1(x).
f(x)=7x+5. Find f−1(x).
f(x)=2x+9. Find f−1(x).
Watch out!
Practice ★★
f(x)=2x+6. Find f−1(10).
f(x)=2x+9. Find f−1(11).
f(x)=3x+1. Find f−1(10).
The point (2,7) is on the graph of f. Tap the point that must be on the graph of f−1.
The point (5,7) is on the graph of f. Tap the point that must be on the graph of f−1.
Exit ticket
Three questions to check you have got it.
f(x)=7x+9. Find f−1(x).
f(x)=2x+2. Find f−1(x).
f(x)=7x+5. Find f−1(x).
Summary card
Inverse functions
Write y=f(x).
Solve for x, undoing the last operation first.
Rename: f−1(x). Check that f(f−1(x))=x.
Watch out: f−1(x) is not f(x)1.
Answers: Functions and their graphs
1. Function notation: evaluate and read f(x)
A
21
24
102
85
51
B
28
66
32
39
C
(2,−1)
(2,3)
D
28
66
32
2. Domain and range in interval notation
A
[9,∞)
[5,∞)
[6,∞)
[4,∞)
B
(−∞,6)∪(6,∞)
(−∞,5)∪(5,∞)
(−∞,3)∪(3,∞)
C
[9,∞)
[6,∞)
[4,∞)
D
[9,∞)
[6,∞)
[4,∞)
3. Piecewise-defined functions and absolute value
A
4
10
8
7
B
15
6
5
10
C
7−x
5−x
D
15
6
5
4. Average rate of change; where a function increases or decreases
A
18
24
7
5
B
9
5
4
C
(−∞,5)
(−∞,1)
(−∞,3)
D
18
24
7
5. Composition of functions
A
16
18
15
13
B
6x+36
5x+17
6x+26
C
1
2
D
16
18
15
6. Transformations: shifts and reflections
A
(x−1)2−6
(x−3)2−1
(x−4)2−6
(x−7)2−1
B
y=(x−1)2−3
y=(x−1)2−1
C
y=f(x)+3 → moves it up 3 units · y=f(x−3) → moves it 3 units to the right · y=−f(x) → reflects it across the x-axis · y=f(−x) → reflects it across the y-axis
D
(x−1)2−6
(x−3)2−1
(x−4)2−6
7. Stretches and compressions; even and odd functions
A
It is squeezed toward the y-axis: horizontal compression by a factor of 4
It is squeezed toward the y-axis: horizontal compression by a factor of 5
It is squeezed toward the y-axis: horizontal compression by a factor of 3