Function notationEvaluating f(x)
Name: Date:
Work out the value each function asks for.
1f(x) = x + 7f(−1) =
2f(x) = 9x − 1f(−1) =
3f(x) = 9x − 13f(5) =
4f(x) = x − 15f(−5) =
5f(x) = −7x + 15f(2) =
6f(x) = x − 13f(8) =
- Free
- Grade 8
- f(x), signed inputs
Evaluating functions worksheets
f(4) is a name for a value, not f times 4. These free printable worksheets evaluate f(x) at a given input, negatives included, so the notation stops looking like multiplication.
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Ready to printEvaluating functions
The answer key prints on a separate page.
What's on this sheet
The whole skill here is the reading. A child who can work out 3x − 5 can stall on f(4) for one reason: it looks like a product. So the first move is always the same — put the input in place of every x, then work it out in the order you already know. Once f(4) reads as the value of the rule when x is 4, the arithmetic is nothing new.
Why this sheet works
The inputs run negative, and that is where the marks are lost. f(−5) with a negative coefficient is where a sign gets dropped, so a good share of the inputs and answers here fall below zero. The input is never 0 or 1 either: f(0) just reads the constant off the rule, and f(1) is the one substitution a child can do without meaning to.
How this one works
One question from this sheet, worked through a step at a time.
f(x) = x + 7; f(−1)
f(x) = x + 7atx = −1
1 × (−1)=−1
−1+7=6
f(−1)=6
- f(−1) is a name for the value of the rule when x = −1. Put −1 where the x is — it is not f times −1.
- Multiply first, sign and all: 1 × (−1) = −1.
- Then add 7: −1 + 7 = 6.
- So f(−1) = 6.