Algebra (all content)
Testing solutions to inequalities
Sal checks if the ordered pairs (3,5) and (1,-7) are solutions of the inequality 5x-3y≥25. Created by Sal Khan and Monterey Institute for Technology and Education.
Want to join the conversation?
- why are these comments from yeaarss ago?(15 votes)
- The pages default to top rank questions and answers, these are often older. If you want to see the new comments, select sort by newest first.(6 votes)
- What Grade would this Math be?(4 votes)
- That's a great question! I would say that it would be 8th grade because it's in Algebra 1! I talked that one to David severin as well!(5 votes)
- how would you graph y>3/2x?(2 votes)
- Since there is no y-intercept you must know that it means that the y-intercept is 0. This means that the line passes through (0,0). So, your equation is now y>3/2x+0. Now graph it and shade. you should have a line that passes through (0,0) with a slope of 3/2 and the shading should be everything above the line. Your welcome and please vote.(7 votes)
- Twenty-six does bigger than twenty-five,but is not equal to it. So why can we say like that?(3 votes)
- The inequation in this video uses the bigger or equal sign, which tests as true if the first number be bigger or equal to the second. In this case, 26 is bigger than 25, so the resulting statement is true.(4 votes)
- What was i suppose to learn about this? i didnt understand a single thing in this video(3 votes)
- When we solve equations or inequalities, we want to make sure our answers are correct. We need to make sure they make sense.
If we have 3, and our inequality is 5z - 2 > 10, we plug the 3 into the inequality to check. It comes out to be 17 > 10. Is this statement true. 3 is an answer to this inequality.(3 votes)
- What do you do if an inequality is (example) x>1 or y>1 with no slope? Which ones go which way?(3 votes)
- y>1 has a slope of 0 and is horizontal (it is parallel to the x-axis). That means you should draw a dotted line at y=1.
x>1 has no slope and is vertical (parallel to the y-axis). That means you draw a dotted line at x=1.(3 votes)
- This video didn't really help. Can anyone attempt to explain please?(4 votes)
- Sure! When given two points (x,y) you substitute the x and y within the equation. For example 2x+3y≥10. And you have the points (4,5) given to you. You replace the x in 2x with point(4) and the y in 3y with point(5), making it look something like this (2*4)+(3*5) ≥10. Then you solve from there, 8+15≥10 = 23≥10.
Hope this helped!(1 vote)
- Do you only switch the inequality signs when two negatives are being divided?(1 vote)
- Well, you are supposed to do that in this case, but it doesn't include everything. When multiplying or dividing in inequalities and a negative number is somewhere, know this:
1. If you are multiplying, and the number that is multiplying (say you have 4 > -7/3 x; the number that I'm talking about would be the reciprocal of x's coefficient [-3/7]) is negative, flip the sign.
2. If the number that is multiplying is positive, don't flip the sign, even if the number multiplied (in the equation above, the number I'm talking about would be 4) is negative.
3. If you are dividing, and the divisor is negative, flip the sign.
4. If the divisor is positive, don't flip the sign, even if the dividend is negative.(5 votes)
- i dont get3:13(3 votes)
- Sal does three things at this point,
1) subtract -5x on both sides
2) divide by -3 on both sides
3) flip the inequality which is required anytime you multiply or divide by a negative number
Does this help or not?(2 votes)
- At1:24, Sal said that is not true that 0 is greater than or equal to 25! Wouldn't he flip the inequality sign?(2 votes)
- No, he can't just flip the inequality. We only flip the inequality if we multiply or divide the entire inequality by a negative value.
The point Sal tested makes the inequality false. This means it is not a solution to the inequality.
Hope this helps.(2 votes)
Determine whether the ordered pairs 3 comma 5 and 1 comma negative 7 are solutions to the inequality 5x minus 3y is greater than or equal to 25. So again, let me just try each of these ordered pairs. We could try what happens when x is equal to 3 and y is equal to 5 in this inequality and see if it satisfies it. And then we could try it for 1 and negative 7. So let's do that first. Let's do it first for 3 and 5. So when x is 3, y is 5. Let's see if this actually gets satisfied. So we get 5 times 3. Let me color code it. So this is 5-- I didn't want to do it in that color-- 5 times 3 minus 3 times 5. Let's see if this is greater than or equal to 25. So 5 times 3 is 15. And then from that, we're going to subtract 15, and let's see if that is greater than or equal to 25. Put that question mark there because we don't know. And 15 minus 15, that is 0. So we get the expression 0 is greater than or equal to 25. This is not true. 0 is less than 25. So this is not true. This is not true. So this ordered pair is not a solution to the inequality. So this is not a solution. You put in x is 3, y is 5, you get 0 is greater than or equal to 25, which is absolutely not true. Now let's try it with 1 and negative 7. So we have 5 times 1 minus 3 times negative 7 needs to be greater than or equal to 25. 5 times 1 is 5, and then minus 3 times negative 7 is negative 21. So it becomes minus negative 21 is to be greater than or equal to 25. This is the same thing as 5 plus 21-- subtracting a negative same thing as adding the positive-- is greater than or equal to 25. And 5 plus 21 is 26 is indeed greater than or equal to 25. So this works out. So this is a solution. And just to see if we can visualize this a little bit better, I'm going to graph this inequality. I'm not going to show you exactly how I do it this time, but I'm going to show you where these points lie relative to this solution. So we have 5-- let me do this is a new color. So we have 5x-- that's not a new color. Having trouble switching colors today. We have 5x minus 3y is greater than or equal to 25. Let me write this inequality in kind of our slope-intercept form. So this would be the same thing. If we subtract 5x from both sides, we get negative 3y is greater than or equal to negative 5x plus 25. I just subtracted 5x from both sides. So that gets eliminated, and you have a negative 5x over here. Now let's divide both sides of this equation, or I should say this inequality, by negative 3. And when you divide both sides of an inequality by a negative number, multiply or divide by a negative number, it swaps the inequality. So if you divide both sides by negative 3, you get y is less than or equal to negative 5 divided by negative 3 is 5 over 3x. And then 25 divided by negative 3 is minus 25/3. So this is now the expression or the inequality, y is less than or equal to 5/3 x minus 25/3. So if I wanted to graph this-- I'll try to draw a relatively rough graph here, but really just so that we can visualize this. So our y-intercept is negative 25/3. That's the same thing as negative 8 and 1/3. So that's 1, 2, 3, 4, 5, 6, 7, 8, and a little bit more than 8. So our y-intercept is negative 8 and 1/3 like that. And it has a slope of 5/3. So that means for every 3 it goes to the right, it rises 5. So it goes 1, 2, 3, it rises 5. So the line is going to look something like this. I'm drawing a very rough version of it. So the line will look something like that, this line over here. That's if it was a y is equal to 5/3 x minus 25/3. But here we have an inequality. It's y is less than or equal to. So for any x, the y's that satisfy it are the y's that equals 5/3 x minus 25/3-- that would be on the line, so it would be that point there-- and all the y's less than it. So the solution is this whole area right over here. Since it's less than or equal to, we can include the line. The equal to allows us to include the line, and the less than tells us we're going to go below the line. And we can verify that by looking at these two points over here. We saw that 3 comma 5 is not part of the solution. So 3 comma 5 is 1, 2-- it's right about there and then up 5. So 3 comma 5 is right above here. It's in this region above the line, and notice not part of the solution. And then 1 comma negative 7 is going to be right over here. It's almost on the line. So 1 comma negative 7 is going to be right over there. But it is, at least, within this solution area. So hopefully that gives you a little bit more sense of how to visualize these things. And we'll cover this in more detail in future videos.