SAT Math : How to find f(x)

Study concepts, example questions & explanations for SAT Math

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Example Questions

Example Question #14 : Algebraic Functions

Given the functions f(x) = 2x + 4 and g(x) = 3x – 6, what is f(g(x)) when = 6?

Possible Answers:

144

12

28

192

16

Correct answer:

28

Explanation:

We need to work from the inside to the outside, so g(6) = 3(6) – 6 = 12.

Then f(g(6)) = 2(12) + 4 = 28.

Example Question #15 : Algebraic Functions

A function f(x) = –1 for all values of x. Another function g(x) = 3x for all values of x. What is g(f(x)) when x = 4?

Possible Answers:

–1

–3

3

–12

12

Correct answer:

–3

Explanation:

We work from the inside out, so we start with the function f(x). f(4) = –1. Then we plug that value into g(x), so g(f(x)) = 3 * (–1) = –3.

Example Question #17 : Algebraic Functions

What is f(–3) if f(x) = x2 + 5?

Possible Answers:

–4

–14

4

15

14

Correct answer:

14

Explanation:

f(–3) = (–3)2 + 5 = 9 + 5 = 14

Example Question #21 : Algebraic Functions

For all values of x, f(x) = 7x2 – 3, and for all values of y, g(y) = 2y + 9. What is g(f(x))?

Possible Answers:

14y2 + 3

2x + 9

14x2 + 3

7y2 – 3

14x2 – 3

Correct answer:

14x2 + 3

Explanation:

The inner function f(x) is like our y-value that we plug into g(y).

g(f(x)) = 2(7x2 – 3) + 9 = 14x2 – 6 + 9 = 14x2 + 3.

Example Question #22 : Algebraic Functions

Find

Possible Answers:

Correct answer:

Explanation:

Simply plug 6 into the equation and don't forget the absolute value at the end.

absolute value = 67

Example Question #41 : Algebraic Functions

An outpost has the supplies to last 2 people for 14 days. How many days will the supplies last for 7 people?

Possible Answers:

\dpi{100} \small 9

\dpi{100} \small 4

\dpi{100} \small 7

\dpi{100} \small 5

\dpi{100} \small 10

Correct answer:

\dpi{100} \small 4

Explanation:

Supplies are used at the rate of \dpi{100} \small \frac{Supplies}{Days\times People}.

Since the total amount of supplies is the same in either case, \dpi{100} \small \frac{1}{14\times 2}=\frac{1}{7\times \ (\&hash;\ of\ days)}.

Solve for days to find that the supplies will last for 4 days.

Example Question #23 : Algebraic Functions

Worker \dpi{100} \small A can make a trinket in 4 hours, Worker \dpi{100} \small B can make a trinket in 2 hours. When they work together, how long will it take them to make a trinket?

Possible Answers:

\dpi{100} \small \frac{1}{2}\ hour

\dpi{100} \small \ 1 \frac{1}{3}\ hours

\dpi{100} \small 6\ hours

\dpi{100} \small \ 1 \frac{1}{2}\ hours

\dpi{100} \small 3\ hours

Correct answer:

\dpi{100} \small \ 1 \frac{1}{3}\ hours

Explanation:

The rates are what needs to be added. Rate \dpi{100} \small A is \dpi{100} \small \frac{1}{4}, or one trinket every 4 hours. Rate \dpi{100} \small B is \dpi{100} \small \frac{1}{2}, one per two hours.

\dpi{100} \small \frac{1}{4}+ \frac{1}{2}=\frac{3}{4}, their combined rate in trinkets per hour.

Now invert the equation to get back to hours per trinket, which is what the question asks for: \dpi{100} \small \frac{4}{3}\ or \ 1 \frac{1}{3}

Example Question #43 : Algebraic Functions

 

Quantity A                  Quantity B  

                               

Possible Answers:

Quantity A and Quantity B are equal

Quantity A is greater

Quantity B is greater

The relationship cannot be determined from the information given.

Correct answer:

Quantity A and Quantity B are equal

Explanation:

Since , then we have that 

and

.

Thus, the two quantities are equal. 

Example Question #44 : Algebraic Functions

If the average of two numbers is \dpi{100} \small 3y and one of the numbers is \dpi{100} \small y+z, what is the other number, in terms of \dpi{100} \small y and \dpi{100} \small z?

Possible Answers:

\dpi{100} \small 3y+z

\dpi{100} \small 5y-z

\dpi{100} \small 5y+z

\dpi{100} \small y+z

\dpi{100} \small 4y-z

Correct answer:

\dpi{100} \small 5y-z

Explanation:

The average is the sum of the terms divided by the number of terms. Here you have \dpi{100} \small y+z and the other number which you can call \dpi{100} \small x. The average of \dpi{100} \small x and \dpi{100} \small y+z is \dpi{100} \small 3y. So \dpi{100} \small 3y=\frac{(x+y+z)}{2}

Multiply both sides by 2.

Solve for \dpi{100} \small x=5y-z.

Example Question #45 : Algebraic Functions

Alice is twice as old as Tom, but four years ago, she was three years older than Tom is now. How old is Tom now?

Possible Answers:

\dpi{100} \small 13

\dpi{100} \small 21

\dpi{100} \small 3

\dpi{100} \small 7

\dpi{100} \small 9

Correct answer:

\dpi{100} \small 7

Explanation:

The qustion can be broken into two equations with two unknows, Alice age \dpi{100} \small (A) and Tom's age \dpi{100} \small (T).

\dpi{100} \small A=2T

\dpi{100} \small A-4=T+3

\dpi{100} \small 2T-4=T+3

\dpi{100} \small T=7

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