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Evaluating Functions
Evaluating a function simply means finding f(x) at
some specific value x. To put it more
bluntly, these are glorified substitution questions. We glorify
them above all because they’re easy. Here’s an example:
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See how that f(5) substituted
a 5 for the x in f(x)?
Well, every time you see an x in the equation,
replace it with a 5:
=52-3=22.gif)
You almost don’t even have to think at all when answering
these questions. If the entire Math section was just a bunch of
evaluating functions questions, amoebas could get 800s and Ivy League
schools would welcome every well-rounded single-celled organism
who applied.
Ah, but life and the new SAT ain’t that easy. Here’s one
wrinkle the new test may throw at you. You may have to evaluate
a function at a variable rather than a constant. For example,
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Okay, slightly harder than substituting in a number, but
still not difficult. Search out all the occurrences of x in
the function and replace it with (x + 1):
=3(x+1).gif)
As long as you remembered to distribute that negative
sign across the (x + 1) to make –x –
1 in that second step, you’re all set.
Performing Operations on Functions
Functions can be added, subtracted, multiplied,
and divided like any other quantities. A few key rules will make
these operations easier. For any two functions f(x)
and g(x),
| Rule | Example | |
|---|---|---|
| Addition |  |
If f(x)
= x2 and g(x)
= 2x: (f + g)(x) = x2 + 2x |
| Subtraction |  |
If f(x)
= x2 + 5 and g(x)
= x2 + 2x +
1: (f – g)(x) = x2 + 5 – x2 – 2x – 1 = –2x + 4 |
| Multiplication |  |
If f(x)
= x and g(x)
= x3 + 8: |
| Division |  |
If f(x)
= 2x and g(x)
= x2: |
Here’s a quick rule to follow for all of these operations
on functions: Work out the value for both functions separately,
and then perform the operation on those two values. Remember that
any time you divide functions,
the resulting function is undefined whenever the g(x) in
the denominator equals zero. Division by zero is always a no-no.
Wacko Symbols Questions (Algebra in Disguise)
The SAT seems to give itself a cooky thrill by creating
odd symbols and then defining those symbols as mathematical functions.
For example, a typical symbol SAT question might say,
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What is the value of 4 @ 2?These symbols questions are just snazzy, dressed-to-kill,
evaluating functions questions. Answer them by plugging in:
Some students get frazzled when they see odd symbols in
their test booklet, which is exactly what the SAT wants. Don’t get
tripped up on these otherwise easy questions.
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