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## Algebra I (2018 edition)

### Course: Algebra I (2018 edition)>Unit 15

Lesson 4: Evaluating expressions with unknown variables

# Worked example: evaluating expressions using structure

Sal solved a few examples where we need to evaluate an expression, but we don't know the individual variable values. Created by Sal Khan.

## Want to join the conversation?

• Hi, I'm in 8th grade and have a cumulative test soon. On my advanced supplement, I came across a fairly hard question (ha, it's probably not very hard for all the smart people on this website :D). I found an answer, but a is in terms of r and b. I will certainly ask my teacher later, but it would be nice and helpful if I could get an explanation here. So: A line L has a slope of -2 and passes through the point (r,-3). A second line, K, is perpendicular to L at (a,b) and passes through the point (6,r). Find a in terms of r. (sorry if this is in the wrong category, but I wasn't entirely sure which one it should be in)
• at you say you can factor out a -9 but why is it a negative? If the original equation is a+b+c = -1 then you would need to factor in a positive 9, right? I'm so confused
• can i just say probably try to rewatch the video and pay attentiion close to what he is saying and watch his steps and if that still doesnt answer your question try to research it
• if the student's still don't understand how can i make it easer for them?
(1 vote)
• Here is what I'd suggest:
Rewatch the videos.
Ask questions (there are people like me who do know this stuff who can help) -- hint, ask very specific questions.
If you still struggle, there are other websites that have educational videos on beginning algebra. There are plenty on YouTube (some are videos of actual teachers teaching a real classroom).
You might see if you can get an Algebra workbook to practice this material with -- Amazon.com and other venues have such workbooks for not very much money.
• where did you get the 5 out of
5a+5b+5C?
• 5 a + 5 b + 5 c
Like Sal explains at around when you rewrite the expression as 5 (a+b+c) its the same thing as 5 a + 5 b + 5 c because of the distributive property.
Hope I helped!
• what if a + 2b + 5c = 7 and 4a + 8b + 20c = d. then what does d = _ ?
• d would be equal to 4a+8b+20c or 4(a+2b+5c) or 28
• So if my question is 2r+3r-4-8=r-8 how am I going to solve that
• How can you tell if the number is a negative or if it is subtracting? For instance, if the problem was:
-9b-9c-7d, how do you know if the 9c and 7d are negatives or being subtracted? In the video, Sal said all the terms were negatives, but how did he know?
(1 vote)
• Sometimes, negative numbers have a "-" sign before it like -1 and subtracting is like - by itself if you see a sentence like.....
-3 - -2 =?
That basically means -3 (-) -2=? so if its like a math test just do a ( ) over the subtracting sign and if you can't tell which is which still look at the number after the negative - 3 which in this case 3 then look at - which is (-) and the last one is -2 so its -3(-)-2= -5!
HOPE IT HELPS ALSO CHECK OUT THIS WEBSITE:

http://www.mathsisfun.com/positive-negative-integers.html
• Hi. At about , Sal shows us that the equation can be solved liked this -
5(7)

So does that mean that a+b+c=7 = a=0, b=0, c=7?
Couldn't a=1, b=1, c=5 or another combination? Thanks.
• You're doing things a little different than Sal.

Sal is solving them with coefficients attached, be they a 7 or a -9. If we know what a, b, and c are, then we can solve it by simply plugging in the numbers. However, if we only know that a + b + c is 7, then we can apply that to all other questions with coefficients in them. In other words, if a, b, and c are unknowns, then we have to solve with the information we have. We are generalizing. In the first half of the video, we see that 5a + 5b + 5c = 35; a, b, and c could be anything as long as they equal 35.

So if they add up, a, b, and c could be anything, really.

Hope that helps!
(1 vote)
• at Sal says he "can factor out and get a -9". Is there a video I missed somewhere here explaining that? I understand factoring, but just not how and why it gets applied in these algebraic expressions. Thanks