Best Digital SAT Math Prep: "Linear Functions"

Frontier Lesson: "Linear Functions" for the Digital SAT

"Linear Functions" is a key topic in the "Algebra" content domain of the SAT! Mastering linear functions is not only essential for the test but also for building a strong foundation in math.


On the Digital SAT, you'll encounter questions about linear functions in multiple-choice and student-produced response (grid-in) formats. The good news? These questions tend to range from easy to medium difficulty, and even the "hard" ones are manageable once you understand the core concepts and tricks.

๐ŸŽฏ Here's what you'll often see:

  • Solving for variables or constants in given linear functions
  • Finding slope, x-intercept, or y-intercept from linear equations
  • Writing linear equations based on word problems or graphs
  • Applying linear relationships to solve real-world problems (e.g., rate, cost, growth)

Definition and Basic Forms of "Linear Functions"

โžข Two Equivalent Forms of "Linear Functions"

  1. Slope-Intercept Form: (most common)

    • : Slope (rate of change), : y-intercept (y-coordinate of the intersection with the y-axis).

    • Example: has a slope and y-intercept .

  2. Standard Form: ( and not both zero)

    • Can be converted to slope-intercept form: (if ).

    • Example: โ†’ .


โžข Key Parameters of "Linear Functions"

(1) Slope (k)

  • Geometric Meaning:

  • Example: A slope means the line rises 1 unit for every 2 units moved to the right.

  • Classification and Graph Characteristics:

    Slope Function TypeGraph DirectionPasses Through Quadrants
    IncreasingUpward to the rightAlways passes through Quadrants I and III
    DecreasingDownward to the rightAlways passes through Quadrants II and IV
    ConstantHorizontal lineParallel to x-axis ()

(2) Intercepts

  • y-intercept (b):
    • Intersection with the y-axis at .
    • Example: has a y-intercept of .
  • x-intercept:
    • Set , solve for (slope-intercept form) or (standard form).
    • Example: has an x-intercept at .

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Graphs and Properties of "Linear Functions"

โžข Graphs and Key Characteristics

  • Graphing Methods:

    • Slope-Intercept Form: Plot the point and another point (e.g., for , calculate ).
    • Standard Form: Find x- and y-intercepts, then connect them (e.g., has intercepts at and ).
  • Symmetry:

    • If (proportional function), the graph is symmetric about the origin (e.g., ).

โžข Special Linear Functions

TypeEquationPropertiesGraph (below)
Proportional FunctionPasses through the origin; slope determines direction (you can review the previous table and graph: for Quadrants I/III; for II/IV)Diagonal line through origin
Constant FunctionHorizontal line; no slope ()Parallel to x-axis
Vertical LineNot a function (undefined slope)Perpendicular to x-axis


โžข Parallel and Perpendicular Lines

1. Parallel Lines

  • Condition: Same slope ().
  • Example:
    • and are parallel.

2. Perpendicular Lines

  • Condition: Slopes satisfy (e.g., one is , the other is ).

  • Example:

    • and are perpendicular.
  • Special Cases:

    • A horizontal line () is perpendicular to any vertical line ().

What Information Is Needed to Write the Equation of A "Linear Function"?

In real Digital SAT, you will encounter two situations:

โžข Given: Slope and a Point

  • Method: Substitute into slope-intercept form and solve for .

Example Steps:

Given slope and point :

  1. Substitute into slope-intercept form:
    โ†’

  2. Final equation:

  • Alternative Point-Slope Method (additional note):
    โ†’ โ†’

โžข Given: Two Points and

Example Steps:

Given points and :

  1. Calculate slope:

  2. Thus, we can have a slope-intercept form

  3. Choose one point (e.g., ) and substitute into the slope-intercept form:
    โ†’

  4. Final equation:

Common SAT "Linear Functions" Question Types

Example 1. Solving for Variables or Constants in Linear Functions

These questions give you a linear function (e.g., ) and ask you to find a missing variable or constant, such as:

  • The function is defined by . What is the value of ?
  • The function is defined by . For what value of does ?
  • If and , what is the value of ?
  • For the linear function , is a constant and . What is the value of ?

What to Do:
Plug the given values into the equation and solve for the unknown. These problems are usually straightforwardโ€”just be careful not to mix up variables (like ) and functions (like ) while calculating. Follow basic algebra steps (add/subtract/multiply/divide) to isolate the variable, and you'll get the answer.

Example:
"If and , what is the value of ?"
โ†’ Replace with :
โ†’ Solve: , so .


๐Ÿ“ฃ Be Careful:

  • Stay organized: Label each term clearly (e.g., slope, constant, input/output).
  • Double-check signs (+/โˆ’) when moving terms across the equation.

Example 2. Finding Slope, x-intercept, or y-intercept

You may need to identify key features of a linear equation, such as:

  • Slope (m): The rate of change (e.g., in , is the slope).
  • y-intercept (b): The point where the line crosses the y-axis (at ).
  • x-intercept: The point where the line crosses the x-axis (found by setting ).

Examples:

  • The function is defined by . What is the y-intercept of the graph of in the -plane?
  • The function is defined as more than times a number . If is graphed in the -plane, what is the best interpretation of the -intercept?

What to Do:

  • Understand the what does "slope/-intercept/-intercept" mean.
  • Rewrite the equation in slope-intercept form () if needed.
  • Identify the intercepts by setting one variable to zero and solving for the other.

Example 3. Defining Linear Equations from Word Problems or Graphs

Some questions present a scenario or a graph and ask you to write the corresponding equation.

  • Sample word problems:

    • A taxi charges a $3 base fee plus $2 per mile. Which equation can be defined for the total cost in terms of miles ?
    • In the -plane, the graph of the linear function contains the point and . Which equation defines , where ?
  • Sample graph questions:

    The graph of the linear function is shown. If and are positive constants, which equation could define ?


What to Do:

  • For giving two points or slope + one point:

    • If given two points and , first calculate the slope ().
    • Then, plug the slope and one point into point-slope form (or plug into the slope-intercept form): .
    • Simplify to slope-intercept form () if needed.
  • For word problems:

    • Identify the variables (the "per" quantity, like cost per mile) and the constant (fixed value, like a base fee).
    • Set up the equation: Total = (Rate ร— Variable) + Initial Value.
    • Example: "A gym charges $30/month plus a $50 sign-up fee" โ†’ .
  • Verify your equation:

    • Plug in a known point to check if it satisfies your equation.
    • For word problems, test a simple case (e.g., "After 0 months, is the cost equal to the initial fee?").

Example 4. Applying Linear Relationships to Real-life Situations Word Problems

1. What These Questions Ask You to Do:

  • Solve for a specific value (e.g., "After 5 hours, how far has the car traveled?")
  • Write a linear equation modeling the situation (e.g., "Which equation defines the total cost after buying tickets?")
  • Interpret a variable/constant (e.g., "..., which of the following is the best interpretation of 55 in this context?")

These problems test your ability to translate real-world scenarios into math and extract meaningful information.


2. Common Real-Life Situations

You'll often encounter linear relationships in these contexts:

  • Speed/Distance/Time:
  • Cost/Revenue/Profit:
  • Temperature Changes:
  • Population/Growth:
  • Money Savings/Earnings:

How to Approach These Problems:

  1. Identify the variables and constantsโ€”what changes ("per hour", "per item") vs. what stays fixed (initial value, base fee).
  2. Set up the equation in form, where:
    • = rate of change (slope)
    • = starting value (y-intercept)
  3. Solve or interpret based on the question's goal.
  4. Double-check units (hours vs. minutes, dollars vs. cents) to avoid mistakes.

Example Question:
A taxi charges a $5 base fare plus $2 per mile. If you paid $21, how many miles did you ride?

  • Rate of change (slope): per mile
  • Constant (y-intercept): is a fixed cost
  • Equation:
  • Solve: โ†’ miles

*Common Trap: Forgetting the base fee and solving โ†’ Wrong answer: miles!

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Cheatsheets During the Actual Digital SAT

๐Ÿš€ Quick Analysis Using Graphs

Given:

โžฅ Determine the sign of the slope by observing the quadrants the line passes through

  • Positive slope: : The line goes from bottom-left to top-right, passing through the first and third quadrants

  • Negative slope: : The line goes from top-left to bottom-right, passing through the second and fourth quadrants

โžฅ Determine the sign of the y-intercept( ) by observing where the line crosses the y-axis

  • Positive y-intercept: : The intersection point is above the x-axis

  • Negative y-intercept: : The intersection point is below the x-axis

  • No y-intercept: : The intersection point passes through the origin


๐Ÿ’ก This method helps you quickly eliminate incorrect answer choices. Sometimes you don't even need to perform actual calculations - you can select the correct answer just by analyzing the graph.

Example:


Explanation:

  • Based on the graph, we can first observe that the function passes through the second and fourth quadrants, which means the slope is negative. At this point, we can immediately eliminate options A and D (both have a slope of 1/3). Next, we only need to choose between B and C.

  • Then, looking at where the line intersects the y-axis, we see it's below the x-axis-that is, on the negative half of the y-axis. This means the y-intercept is negative, so option B is also eliminated (its y-intercept is +6).

  • Thus, the correct answer is C.

In this process, we didn't pick points on the graph and calculate the function's equation to solve it. We could determine the answer just by "looking at the graph directly"!


๐Ÿš€ Quick Calculation of Slope

  • Master the slope formula for two points and :
  • Do not swap the numerator and denominator.
  • If the SAT problem provides two intercept points (e.g., and ), the slope is:
    .

๐Ÿš€ Intercept Shortcut Method

  • Memorize these conclusions to save time during exams:

    • x-intercept (derived by setting in )
    • y-intercept (obtained directly by setting )
  • Note: If the equation is in standard form , then:

    • x-intercept
    • y-intercept (no rearrangement needed).

๐Ÿš€ Two-Point Form to Slope-Intercept Form

  • Given two points, first calculate the slope , then use the point-slope form:

    Finally, rearrange it to:
    .

๐Ÿš€ Quick Graph Reading

  • y-intercept: Observe the y-coordinate where the line crosses the y-axis.
  • Slope: Count the grid units for (e.g., a point moves right 2 units and up 1 unit to return to the line โ†’ ).

๐Ÿš€ Keyword Translation in Word Problems

  • Terms like "Initial value", "base fee", "fixed cost", etc. โ†’ y-intercept .
  • Terms like "Rate of change", ".. per ..", etc. โ†’ Slope .

๐Ÿš€ Extreme Value Verification

  • Test or to validate the equation's reasonability.

๐Ÿš€ Reverse Substitution for Multiple-Choice

  • For complex functions (e.g., fractions), if the question asks for , substitute the given options backward to check which fits the function.

๐Ÿš€ Simplify Equations

  • First, check if the function can be simplified (e.g., simplifies to ). This makes analysis clearer and reduces calculation difficulty.

Quick Practice: Test Your Skills!

Question 1

In the -plane, the graph of the linear function passes through the points and . Which of the following defines?

A)

B)

C)

D)


Explanation:

  • Step 1: Find the slope using the formula :

Thus, Option A and option C can be eliminated.

  • Step 2: Use the point-slope form with one of the points, let's use :

  • Or, you can substitute one point, for example , to either option B or option D to check if the result is correct. Let's test in option B:

, which matches the given point .

  • Finally, the correct answer is B

Question 2

A worker repairs wooden boxes and mows the lawn. The formula relates the total time , in minutes, it takes to repair wooden boxes and mow the lawn. Which of the following describes the meaning of the in this context?

A) The increase in time, in minutes, it takes the worker to repair a wooden box.

B) The time, in minutes it takes the worker to mow the lawn.

C) The increase in time, in minutes, it takes the worker to repair a wooden box and mow the lawn.

D) The total time, in minutes, it takes the worker to repair the wooden boxes.


Explanation:

  • The equation represents a linear function. In this context:
    • is a variable and represents the number of wooden boxes repaired.
    • is rate of change and represents the time spent repairing each box.
    • is a fixed value. It also represents the y-intercept of a linear function, which is the value of when (meaning no boxes are repaired). This constant time would be related to something other than repairing boxes, and the only activity mentioned is mowing the lawn.

Thus, The correct answer is B.

Question 3


Explanation:

  • Step 1: Determine the sign of the slope
    From the graph, we can notice that the line passes through the second and fourth quadrants, which means the slope must be negative. So we can quickly eliminate option A and option B (positive slope and )
  • Step 2: We can see that option C and option D have the same -intercept: , so we need to further determine the value of slope. Let = slope and the equation can be written as
  • Step 3: Plug a point into the equation.
    From the graph, we can see the point is on the line. Let's solve for :
    • Multiply both sides by 4 to remove the denominator:
  • Thus, the function is which matches option D

Question 4

A car rental company charges a flat fee of $ plus $ per mile driven. If Jamal paid $ for his rental, how many miles did he drive?

A) miles
B) miles
C) miles
D) miles


Explanation:

  • Step 1: Write the linear function for the cost. Let = number of miles driven and = total cost.
    • Rate of change (slope): per mile driven
    • Constant (y-intercept): is a fixed cost
    • Equation:
  • Step 2: Set up an equation using the given information:
  • Step 3: Solve for

Thus, Jamal drove 300 miles, which matches option A.

Question 5

129
224
319
414

For the linear function , the table shows four values of and their corresponding values of . The function can be written as , where and are constants. What is the value of ?


Explanation:

  • Step 1: Determine the slope ()
    The slope of a linear function can be calculated using any two points from the table. Let's use the first two points and :

    So, the slope is .

  • Step 2: Find the y-intercept ()
    Now that we have , we can plug one of the points into the equation to solve for . Let's use the first point :

  • Thus,the value of is

Your Turn! Realistic "Linear Functions" Questions for DSAT Success

Question 1

Difficulty level: Easy

For the linear function is a constant and f(7) = 35. What is the value of b?

A).

B).

C).

D).

Question 2

Difficulty level: Medium

The function is defined as more than times a number . If is graphed in the -plane, what is the best interpretation of the -intercept?

A). When , the number is .

B). When the number is , .

C). For each increase of in the value of the number, increases by .

D). The value of increases by for each increase of in the value of the number.

Question 3

Difficulty level: Hard

The graph of the linear function is shown. If and are positive constants, which equation could define ?

A).

B).

C).

D).

"Linear Functions" Learning Checklist

  • ๐Ÿ”˜ Know two equivalent forms of a linear function:

    • Slope-Intercept Form: most common
    • Standard Form: ( and not both zero)
  • ๐Ÿ”˜ Be able to correctly identify the slope, -intercept, -intercept from a linear equation.

  • ๐Ÿ”˜ Can write and convert between different forms of linear equations (slope-intercept, point-slope, standard form).

  • ๐Ÿ”˜ Understand special linear functions:

    • Proportional Function
    • Constant Function
    • Vertical Line(Not a function)
  • ๐Ÿ”˜ Know how to calculate the slope between two points:

  • ๐Ÿ”˜ Be able to write the equation of a line given various information:

    • slope and y-intercept
    • slope and a point
    • two points
  • ๐Ÿ”˜ Be familiar with the common "Linear Functions" question types in real Digital SAT:

    • Solve for variables or constants
    • Find slope, -intercept, or -intercept
    • Define a linear equation from words or a graph
    • Apply linear relationships to real-life situations
  • ๐Ÿ”˜ Master practical strategies to solve questions faster:

    • Determine the sign of the slope by observing the quadrants the line passes through
    • Determine the y-intercept by observing where the line crosses the y-axis
    • Calculate the value of slope in a more efficient way
    • Memorize conclusions to slove for the -intercept and -intercept
    • Use point-slope form to get the needed slope-intercept form of a linear function
    • Count the grid units to obtain the slope
    • Translate the keywords in proper way in word questions
    • Test the equation quickly by using extreme values
    • Substitute the given options backward to check if they fit the given function.
    • Always simplify the equation to reduce calculation difficulty.

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