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A compound inequality with no solution

Sal solves the compound inequality 5x-3<12 AND 4x+1>25, only to realize there's no x-value that makes both inequalities true. Created by Sal Khan and Monterey Institute for Technology and Education.

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  • aqualine seed style avatar for user Coco
    Hi! So my question is more so regarding the questions section that you usually do to test yourself after watching the videos. I’ve been trying to finish it with a perfect score for the past two days but I simply do not get the thinking behind the answer choices. I know how to solve the inequality, I know how to graph it, but when it asks me to pick the right answer between both solutions I become completely confused! Not to mention the other answer choices such as: solution for inequality A, solution for inequality B, solution for both, “All x’s are right”, or “no solution” the answer always surprises me and the hint section is not helping. Would someone explain to me how to get past it? Would it be possible for Sal to make a short video on how to solve the questions and pick between those answers?

    Thank you and sorry for the lengthy post!
    (29 votes)
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    • stelly blue style avatar for user Kim Seidel
      Sounds like you are getting confused when you have to figure out the intersection or the union of the 2 inequalities. There is a video on intersections and unions of sets. This might help you understand the basic concept of intersections and unions. It is at this link: https://www.khanacademy.org/math/statistics-probability/probability-library/basic-set-ops/v/intersection-and-union-of-sets

      The easiest way I find to do the intersection or the union of the 2 inequalities is to graph both.
      If the compound inequality is "and", you need to find the intersection. The intersection is where the values of the 2 inequalities overlap. For example:
      -- graph x > -2 and x < 5. These overlap from -2 up to 5. The intersection is: -2 < x < 5; or in interval notation: (-2, 5)
      -- graph x > -2 and x > 5. These 2 inequalities overlap for all values larger than 5. The intersection is: x>5; or in interval notation: (5, infinity)
      -- graph x > -2 and x < -5. These 2 inequalities have no overlap. So, there is no intersection. This is the case that results in No Solution.

      If the compound inequality is "or", you need to find the union. The union of the 2 inequalities is a new set that contains all values from both sets combined. For example:
      -- graph x > -2 or x < -5. The 2 inequalities have completely separate graphs. All values from both graphs become the solution: x > -2 or x < -5; or in interval notation: (-infinity, -5) or (-2, infinity)
      -- graph x > -2 or x > 5. The graphs of the inequalities go in the same direction. We need a set that includes all values for both inequalities. This would be the longer graph. So, the solution is: x > -2; or in interval notation: (-2, infinity)
      -- graph x > -2 or x < 5. These overlap -- so the union of the 2 sets would encompass the entire number line. This is the scenario that become All Real Numbers or All values of X are solutions.

      hope this helps.
      (32 votes)
  • duskpin ultimate style avatar for user Rosa
    Is it possible to graph a no solution inequality on the number line? If so, how?
    (6 votes)
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  • hopper cool style avatar for user Johnathon Aaron
    Sal states that there is no solution, but what if x was a function of some sorts or a liner equation with multiple places on the number line that fall into the constraints both less then 3 and greater than 6? example, a solution set of (2,7)
    (6 votes)
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  • hopper jumping style avatar for user littlesisiscool
    How do you eliminate options in the problems. What is the difference between AND and OR? I am REALLY struggling with this concept. Its like math block. I feel like I've never struggled more with a concept than this one. AAAH! Please help.
    (6 votes)
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    • stelly blue style avatar for user Kim Seidel
      The word AND tells you to find the intersection of both solution sets. An intersection is the solutions in common, or that overlab.

      The word OR tells you to find the union of the 2 solution sets. A union is 2 sets combine all possible solutions from both sets.

      To learn more about these, search for "intersection and union of sets". There is a video on KA that walks you thru them.
      (7 votes)
  • blobby green style avatar for user coryhunnicutt1
    my question is whats the point of this. when will i use this in the real world lmao
    (9 votes)
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  • leafers ultimate style avatar for user Dooder
    Can there be an OR inequality that has no solutions?
    (4 votes)
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  • starky sapling style avatar for user Izzy
    how do you choose an answer to an 'or' compound inequality? Since you get two answers, how do you know which one is correct?
    with lots of thanks,
    Izzy.
    (2 votes)
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    • stelly blue style avatar for user Kim Seidel
      The word OR tells you to find the union of the 2 solutions sets from the individual inequalities. It combines both sets into one. As long as a solution works in at least one of the inequalities, then it is a solution to the compound inequality. You don't pick one set of answers over the other.
      Hope this helps.
      (8 votes)
  • duskpin seedling style avatar for user Isabella  Braquet
    how do you know when to switch the inequality symbol?
    (3 votes)
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  • starky sapling style avatar for user Gabriel870
    At that point couldn't you bend the number line like you can bend space?
    I know you can't, but still.
    (5 votes)
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  • blobby green style avatar for user Kirsten Cress
    Why didn't he flip the inequalities when dividing on both sides??
    (3 votes)
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Video transcript

Solve for x, 5x - 3 is less than 12 "and" 4x plus 1 is greater than 25. So let's just solve for X in each of these constraints and keep in mind that any x has to satisfy both of them because it's an "and" over here so first we have this 5 x minus 3 is less than 12 so if we want to isolate the x we can get rid of this negative 3 here by adding 3 to both sides so let's add 3 to both sides of this inequality. The left-hand side, we're just left with a 5x, the minus 3 and the plus 3 cancel out. 5x is less than 12 plus 3 is 15. Now we can divide both sides by positive 5, that won't swap the inequality since 5 is positive. So we divide both sides by positive 5 and we are left with just from this constraint that x is less than 15 over 5, which is 3. So that constraint over here. But we have the second constraint as well. We have this one, we have 4x plus 1 is greater than 25. So very similarly we can subtract one from both sides to get rid of that one on the left-hand side. And we get 4x, the ones cancel out. is greater than 25 minus one is 24. Divide both sides by positive 4 Don't have to do anything to the inequality since it's a positive number. And we get x is greater than 24 over 4 is 6. And remember there was that "and" over here. We have this "and". So x has to be less than 3 "and" x has to be greater than 6. So already your brain might be realizing that this is a little bit strange. This first constraint says that x needs to be less than 3 so this is 3 on the number line. We're saying x has to be less than 3 so it has to be in this shaded area right over there. This second constraint says that x has to be greater than 6. So if this is 6 over here, it says that x has to greater than 6. It can't even include 6. And since we have this "and" here. The only x-es that are a solution for this compound inequality are the ones that satisfy both. The ones that are in the overlap of their solution set. But when you look at it right over here it's clear that there is no overlap. There is no x that is both greater than 6 "and" less than 3. So in this situation we have no solution.