Inequalities & Absolute Value - SAT Math

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Question

If and , which of the following must be true?

Answer

Systems of inequalities can be solved just like systems of equations, but with three important caveats:

  1. You can only use the Elimination Method, not the Substitution Method. Since you only solve for ranges in inequalities (e.g. a < 5) and not for exact numbers (e.g. a = 5), you can't make a direct number-for-variable substitution.

  2. In order to combine inequalities, the inequality signs must be pointed in the same direction.

  3. When you're combining inequalities, you should always add, and never subtract. Since subtraction of inequalities is akin to multiplying by -1 and adding, this causes errors with flipped signs and negated terms. Always look to add inequalities when you attempt to combine them.

With all of that in mind, here you can stack these two inequalities and add them together:

Notice that the terms cancel, and that with on top and on bottom you're left with only one variable, . The new inequality hands you the answer, .

Note that if this were to appear on the calculator-allowed section, you could just graph the inequalities and look for their overlap to use process of elimination on the answer choices. These two inequalities intersect at the point (15, 39).

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Question

Based on the system of inequalities above, which of the following must be true?

Answer

Systems of inequalities can be solved just like systems of equations, but with three important caveats:

  1. You can only use the Elimination Method, not the Substitution Method. Since you only solve for ranges in inequalities (e.g. a < 5) and not for exact numbers (e.g. a = 5), you can't make a direct number-for-variable substitution.

  2. In order to combine inequalities, the inequality signs must be pointed in the same direction.

  3. When you're combining inequalities, you should always add, and never subtract. Since subtraction of inequalities is akin to multiplying by -1 and adding, this causes errors with flipped signs and negated terms. Always look to add inequalities when you attempt to combine them.

With all of that in mind, you can add these two inequalities together to get:

So . This matches an answer choice, so you're done. Note that process of elimination is hard here, given that is always a positive variable on the "greater than" side of the inequality, meaning it can be as large as you want it to be. And as long as is larger than , can be extremely large or extremely small. This is why systems of inequalities problems are best solved through algebra; the possibilities can be endless trying to visualize numbers, but the algebra will help you find the direct, known limits.

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Question

If x > r and y < s, which of the following must also be true?

Answer

Because of all the variables here, many students are tempted to pick their own numbers to try to prove or disprove each answer choice. But an important technique for dealing with systems of inequalities involves treating them almost exactly like you would systems of equations, just with three important caveats:

  1. You can only use the Elimination Method, not the Substitution Method. Since you only solve for ranges in inequalities (e.g. a < 5) and not for exact numbers (e.g. a = 5), you can't make a direct number-for-variable substitution.

  2. In order to combine inequalities, the inequality signs must be pointed in the same direction.

  3. When you're combining inequalities, you should always add, and never subtract. Since subtraction of inequalities is akin to multiplying by -1 and adding, this causes errors with flipped signs and negated terms. Always look to add inequalities when you attempt to combine them.

Here, the first step is to get the signs pointing in the same direction. You already have x > r, so flip the other inequality to get s > y (which is the same thing − you’re not actually manipulating it; if y is less than s, then of course s is greater than y). Now you have:

x > r

s > y

And you can add the inequalities:

x + s > r + y

That’s similar to but not exactly like an answer choice, so now look at the other answer choices. Two of them involve the x and y term on one side and the s and r term on the other, so you can then subtract the same variables (y and s) from each side to arrive at:

x - y > r - s

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Question

If and , which of the following must be true?

Answer

Systems of inequalities can be solved just like systems of equations, but with three important caveats:

  1. You can only use the Elimination Method, not the Substitution Method. Since you only solve for ranges in inequalities (e.g. a < 5) and not for exact numbers (e.g. a = 5), you can't make a direct number-for-variable substitution.

  2. In order to combine inequalities, the inequality signs must be pointed in the same direction.

  3. When you're combining inequalities, you should always add, and never subtract. Since subtraction of inequalities is akin to multiplying by -1 and adding, this causes errors with flipped signs and negated terms. Always look to add inequalities when you attempt to combine them.

So what does that mean for you here? You have two inequalities, one dealing with and one dealing with . But all of your answer choices are one equality with both and in the comparison. Since your given inequalities are both "greater than," meaning the signs are pointing in the same direction, you can add those two inequalities together:

Sums to:

And now you can just divide both sides by 3, and you have:

Which matches an answer choice and is therefore your correct answer.

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Question

Which of the following consists of the -coordinates of all the points that satisfy the system of inequalities above?

Answer

Systems of inequalities can be solved just like systems of equations, but with three important caveats:

  1. You can only use the Elimination Method, not the Substitution Method. Since you only solve for ranges in inequalities (e.g. a < 5) and not for exact numbers (e.g. a = 5), you can't make a direct number-for-variable substitution.

  2. In order to combine inequalities, the inequality signs must be pointed in the same direction.

  3. When you're combining inequalities, you should always add, and never subtract. Since subtraction of inequalities is akin to multiplying by -1 and adding, this causes errors with flipped signs and negated terms. Always look to add inequalities when you attempt to combine them.

Here you have the signs pointing in the same direction, but you don't have the same coefficients for in order to eliminate it to be left with only terms (which is your goal, since you're being asked to solve for a range for ). So you will want to multiply the second inequality by 3 so that the coefficients match. That yields:

When you then stack the two inequalities and sum them, you have:

Yields:

You can then divide both sides by 4 to get your answer:

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Question

Which of the following consists of the -coordinates of all of the points that satisfy the system of inequalities above?

Answer

This systems of inequalities problem rewards you for creative algebra that allows for the transitive property. You know that , and since you're being asked about you want to get as much value out of that statement as you can. And while you don't know exactly what is, the second inequality does tell you about . Note that algebra allows you to add (or subtract) the same thing to both sides of an inequality, so if you want to learn more about , you can just add to both sides of that second inequality. In doing so, you'll find that becomes , or .

Now you have two inequalities that each involve . If and , then by the transitive property, .

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Question

Which of the following represents the complete set of values for that satisfy the system of inequalities above?

Answer

Systems of inequalities can be solved just like systems of equations, but with three important caveats:

  1. You can only use the Elimination Method, not the Substitution Method. Since you only solve for ranges in inequalities (e.g. a < 5) and not for exact numbers (e.g. a = 5), you can't make a direct number-for-variable substitution.

  2. In order to combine inequalities, the inequality signs must be pointed in the same direction.

  3. When you're combining inequalities, you should always add, and never subtract. Since subtraction of inequalities is akin to multiplying by -1 and adding, this causes errors with flipped signs and negated terms. Always look to add inequalities when you attempt to combine them.

Here you should see that the terms have the same coefficient (2), meaning that if you can move them to the same side of their respective inequalities, you'll be able to combine the inequalities and eliminate the variable. To do so, subtract from both sides of the second inequality, making the system:

(the first, unchanged inequality)

(the new second inequality)

When you sum these inequalities, you're left with:

Here is where you need to remember an important rule about inequalities: if you multiply or divide by a negative, you must flip the sign. So to divide by -2 to isolate , you will have to flip the sign:

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Question

If and , which of the following must be true?

Answer

When students face abstract inequality problems, they often pick numbers to test outcomes. But that can be time-consuming and confusing - notice that with so many variables and each given inequality including subtraction, you'd have to consider the possibilities of positive and negative numbers for each, numbers that are close together vs. far apart. There are lots of options.

The more direct way to solve features performing algebra. Notice that with two steps of algebra, you can get both inequalities in the same terms, of . If you add to both sides of you get:

And if you add to both sides of you get:

If you then combine the inequalities you know that and , so it must be true that .

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Question

Which of the following is a possible value of x given the system of inequalities below?

3x+4y > 54

-4x+2y < 10

Answer

In order to combine this system of inequalities, we’ll want to get our signs pointing the same direction, so that we’re able to add the inequalities. We’re also trying to solve for the range of x in the inequality, so we’ll want to be able to eliminate our other unknown, y. In order to accomplish both of these tasks in one step, we can multiply both signs of the second inequality by -2, giving us

3x+4y > 54 (our original first inequality)

8x-4y > -10 (our new, manipulated second inequality)

We can now add the inequalities, since our signs are the same direction (and when I start with something larger and add something larger to it, the end result will universally be larger) to arrive at

3x+4y > 54

+8x-4y > -10

11x > 44

Thus, dividing by 11 gets us to

x > 4. Only positive 5 complies with this simplified inequality.

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Question

Which of the following set of coordinates is within the graphed solution set for the system of inequalities below?

x+2y > 16

-3x+y < 1

Answer

In order to combine this system of inequalities, we’ll want to get our signs pointing the same direction, so that we’re able to add the inequalities. We’ll also want to be able to eliminate one of our variables. In order to do so, we can multiply both sides of our second equation by -2, arriving at

x+2y > 16 (our original first inequality)

6x- 2y > -2 (our new, manipulated second inequality)

Adding these inequalities gets us to

x+2y > 16

+6x- 2y > -2

7x > 14

Dividing this inequality by 7 gets us to

x > 2.

Thus, the only possible value for x in the given coordinates is 3, in the coordinate set (3, 8), our correct answer.

Note - if you encounter an example like this one in the calculator-friendly section, you can graph the system of inequalities and see which set applies. The graph will, in this case, look like:

And we can see that the point (3, 8) falls into the overlap of both inequalities. We could also test both inequalities to see if the results comply with the set of numbers, but would likely need to invest more time in such an approach. Here, drawing conclusions on the basis of x is likely the easiest no-calculator way to go!

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Question

Which of the following, if solved, would yield the set of all real numbers that are exactly 15 units away from -3?

Answer

If you're looking for a number exactly units to either side of , you need an absolute value equation that gives you answers of and .

One way forward is to simply solve each equation for using the two-case method, assuming first that the expression within the absolute value signs positive and then that it is negative and solving the resulting equation for each case.

If you do so, choice becomes: , or and , or , which is correct.

Choice becomes: , or , which doesn't work.

Choice becomes: , or , which doesn't match.

Choice becomes: , or , which doesn't work.

You could also quickly solve this question if you remember the general way number line absolute value problems like this one work. Each one will be in the general form , where the "Distance" referenced is the distance from the center value to one of the two values for .

So in this case the "center value" would be and the "distance" would be , yielding an equation of .

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Question

If is an integer, what is the value of such that is minimized?

Answer

Any time you have an absolute value equation and a question that is asking for the smallest possible value for the expression, remember that absolute value expressions can be zero but can never be less than 0. There are two ways to solve a problem like the one given. The first is by finding the value of such that the expression within the absolute value signs is zero and then finding the closest integer to that number. The second is by inspection (simply using brute force to find the value for xx that will minimize the value given).

If you set , and that . Since , the closest integer value is , which will yield a value of .

The other way to approach this problem is by inspection, or through brute force. Try plugging in a few values for to see what happens.

If then the expression is

If , then the expression is .

If x=2x=2 then the expression becomes .

Notice that the function decreases and then increases around , indicating that (for integer values, at least) will yield the smallest result for .

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Question

Which of the following equations describes the set of all real numbers that are units away from ?

Answer

This problem is a perfect candidate to test the answer choices, as the calculations required are not particularly difficult but the algebraic setup can be quite challenging to conceptualize. If you look for the two numbers that are 66 units away from −4−4, they are:

Then plug those values in as and see which equations work. For answer choice , does equal , and also equals , so satisfies the equation.

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Question

Which of the following equations describes the set of all real numbers that are 2 units away from -3?

Answer

Even if you're unsure of where to start on this problem, you should have a head start. The problem is testing absolute values, and you should know that the result of any absolute value is always nonnegative, . So the answer choices that include an absolute value equalling a negative number must be incorrect: that just cannot be possible.

To test the remaining choices, consider that the numbers that are exactly two units away from -3 are -3+2 = -1, and -3-2 = -5. When you plug these numbers in for in the answer choices, only one is valid:

gives you:

-->

-->

Therefore this absolute value satisfies the given situation, and is correct.

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Question

For what value of is ?

Answer

It is important to recognize that absolute values must be nonnegative, . That means that for this given expression, the can only go as low as , and then the second part of the expression asks you to add . So this expression can never equal zero: it's an absolute value added to 1, so the lowest this expression can be is 1.

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Question

Which of the following choices provides the complete solution set for the equation ?

Answer

To solve an absolute value like this, recognize that there are two outcomes inside an absolute value that would have it equal 3. If the inside of an absolute value expression is 3, then the result is 3. Or if the inside of an absolute value expression equals -3, the absolute value will equal 3. So you can solve this as two equations:

and .

Solving for the first one, you have:

And solving for the second one, you have:

Therefore, the correct answer is:

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Question

What is the sum of the two solutions to the equation ?

Answer

To solve an equation with an absolute value, recognize that there are two things that would make an absolute value equal 5: the inside of the absolute value could equal 5, or it could equal -5. So your job is to solve for both possibilities:

Possibility 1:

You can then subtract 3 from both sides to get:

And divide both sides by 2 to finish:

Possibility 2:

Subtract 3 from both sides to get:

Divide both sides by 2:

Your two solutions are 1 and -4. Since the question wants the sum of the two answers, you can add them together to get your answer, -3.

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Question

What is the sum of all real-number solutions to the equation ?

Answer

When you're solving equations involving absolute values, it's important to recognize that there are generally two solutions. Here if the inside of the absolute value equals 1, you've solved for -- or if the inside of the absolute value equals -1 you've also solved for . So you should solve this as two equations:

Possibility 1

Here you can add 7 to both sides to get:

And then divide both sides by 2:

Possibility 2

Add 7 to both sides:

And divide both sides by 2:

The question asks for the sum of all answers, so add to get the right answer,

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Question

For which value of is the equation true?

Answer

An important thing to know about absolute values is that their minimum value is zero; absolute values must be nonnegative. So here if you take the result of an absolute value and then add 1 to it, it simply cannot equal 0. To do so, the absolute value itself would have to equal -1, and that is just not possible.

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Question

Which of the following equations describes the set of all real numbers that are 5 units away from -3?

Answer

One helpful shortcut on this problem is just understanding that the result of an absolute value can never be negative. So an answer choice like simply cannot be correct: it's not a valid equation.

Of course, there are three remaining choices so your guessing probability isn't high enough to quit now. To solve this, first think about which values are 5 units away from -3. Since -3 + 5 = 2, and -3 - 5 = -8, you have two values that you know fit the definition: 2 and -8.

Now plug those numbers into the answer choices to see which fit with one of the absolute values. You'll see that fits:

, so this satisfies

, so this satisfies

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