About this video
Sal works through examples of writing expressions with variables: '11 more than a' as a + 11, 'the sum of d and 9' as d + 9, j - 15, 7 times r as 7·r or 7r, 10 times u, and 8 divided by d as 8/d. He explains that in algebra a raised dot is used for multiplication because the cross sign can be confused with the variable x.
You'll be able to
- turn words like 'more than', 'sum', 'times' and 'divided by' into an expression with a variable.
- write multiplication with a variable as 7r or 7·r.
- explain why algebra uses a dot instead of a cross for multiplication.
Key words
- variable
- A letter, like a or r, that stands for a number.
- expression
- A maths phrase made of numbers, variables and operations, like a + 11 or 7r.
- sum
- The answer when you add things together.
Transcript
- [Voiceover] Let's do some examples of the writing expressions with variables exercise. So it says "Write an expression to represent 11 more than a." Well you could just have a but if you want 11 more than a, you would wanna add 11 so you could write that as a plus 11.
You could also write that as 11 plus a. Both of them would be 11 more than a. So let's check our answer here. We got it right. Let's do a few more of these. "Write an expression to represent the sum of d and 9." So the sum of d and 9, that means you're gonna add d and 9.
So I could write that as d plus 9 or I could write that as 9 plus d. And check our answer. Got that right. Let's do a few more of these. "Write an expression to represent j minus 15." Well, I could just write it with math symbols instead of writing the word minus.
Instead of writing M-I-N-U-S, I could write j minus 15. And then I check my answer. Got it right. Let's do a few more of these. This is a lot of fun. "Write an expression to represent 7 times r." There's a couple ways I could do it. I could use this little dot right over here, do 7 times r like that.
That would be correct. I could literally just write 7r. If I just wrote 7r that would also count. Let me check my answer. That's right. Let me do a couple of other of these just so you can see that I could've just done 10 and this is not a decimal, it sits a little bit higher than a decimal.
It's multiplication and the reason why once you start doing algebra, you use this symbol instead of that kind of cross for multiplication is that x-looking thing gets confused with x when you're using x as a variable so that's why this is a lot more useful. So we wanna write 10 times u, 10 times u, let's check our answer.
We got it right. Let's do one more. "Write an expression to represent 8 divided by d." So we could write it as 8 and then I could write a slash like that, 8 divided by d. And there you go. This is 8 divided by d. Let me check, let me check the answer. I'll do one more of these.
Oh, it's 6 divided by b. Alright, same thing. So 6, I could use this tool right over here. It does the same thing as if I were to press the backslash. So 6 divided by b. Check my answer. We got it right.
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