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### Course: 6th grade (Illustrative Mathematics) > Unit 6

Lesson 15: Lesson 15: Equivalent exponential expressions# Evaluating expressions with variables: exponents

In this math lesson, we learn to evaluate an algebraic expression with exponents by following the order of operations (PEMDAS). We substitute a given value for the variable, calculate exponents, perform multiplication, and finally, subtraction. By applying these steps, we successfully find the value of the expression. Created by Sal Khan and Monterey Institute for Technology and Education.

## Want to join the conversation?

- how do you do a problem when they don' t give you what a letter is?

Like this 62 A13-2 10(13 votes)- It is not possible to algebraically solve a problem if they do not give you the value of the letter. The best you can do is to simplify the equation, which would mean that you use PEMDAS on all of the numbers, and the variable would have to stay. However, if the number set you gave were to equal something , then it would be possible to find A, but without that, you cannot solve a problem if they do not give you what the letter is. Hope this helps :) ask if you have any other questions!(36 votes)

- At around 51 seconds Sal tells us that addition and subtraction and multiplication and division are at the same level then why is it PEMDAS not PEDMSA(3 votes)
- PEMDAS has 4 rules, not 6.

1) P = Parentheses. Do all work inside the parentheses as your 1st step

2) E = Exponents. All exponents come next

3) MD = Multiply & divide. These are in the same rule. You always work left to right within the rule. Example: 8/2*6 = 4*6 = 24. Division is done 1st because it is on the left.

4) AS = Add & Subtract. Again, these are in the same rule. You need to work left to right.

So, Sal's statement is correct. He is trying to explain / remind us that there are only 4 rules, not 6.

Hope this helps.(21 votes)

- At0:31, Sal replaces the Y into the equation. What is the logic behind NOT taking all of 5y to the fourth?(6 votes)
- The proper notation to take 5y to the fourth power would be (5y)^4

The number in the video represents 5y^4, meaning you raise the variable(y) to the fourth power, then multiply by 5.

Was this answer clear enough?(5 votes)

- PEMDAS can be remembered as

Parentheses ( ) Please

Exponents 2* Excuse

Multiplication x My

Division / Dear

Addition + Aunt

Subtraction - Sally

or if your older some teachers recommend GEMDAS same thing but the "G" stands grouping symbols "[]" and then parentheses

3* 4+[ 89 + 3(56+3) * (-5%)

A real life problem above; see if you can solve !

Hope this helps !(10 votes)- Your problem has unequal brackets, so please correct. you open a bracket and two parentheses, you close two parentheses, but you never show where you close your bracket, so this presents a dilemma to me on exactly how to solve it.(3 votes)

- In evaluating 5y^4-y^2= 5(3)^4-3^2, how come the negative sign was not considered when computing for 3^2 instead of being -3^2? This confused me as I assumed since the negative symbol before a number would require to evaluate it as a negative. Doing that would give an answer of 414 and not 396 since the equation would change to 5(81)+9 no?(7 votes)
- There is a difference between -3^2 which is -9 and (-3)^2 which is 9. So this minus has to do with the term, not a part of the term.(7 votes)

- 40+40x0+1 =

I'm doing it like this 40+(40x0)+1 =41 is this correct(5 votes)- yes, in order to make sure you answer it correctly you follow the simple rule:

P ( )

E exponents

M multiply

D divide

A add

S subtract

solve the problem in that order(7 votes)

- I need help with a question that doesn't have anything to do with topic but it would be nice if someone helped me.

A wetsuit cost one third of the cost of diving gear. David hires a wetsuit and diving gear for the same time and length and pays $144 in total.

How much money did david spend on the wetsuit in dollars.(6 votes)- thank you so much i got i got $216 for the diving suit is that correct?(4 votes)

- For quiz/practice questions students really should be told how Khan would like the answers. Do they want fractions or do they want decimal answers? After several "wrong" answers to a question we figured out they wanted a fraction.(4 votes)
- usually they want it as the fraction/decimal is displayed in the problem. For example if they use decimals in the equations or title of the practice then most likely it will want decimal answers. I know this can be frustrating especially the equations with both, but it will allow you to retake the quiz so you can get the right answers and a good score

Hope this helps!(7 votes)

- What can i do to pass all my 9th grade classes(1 vote)
- I think you just have to work hard, attend classes, and study.(7 votes)

- Why do we need to have variables when we are just going to write them in right after? It makes no sense.(3 votes)
- These are probably just training you to learn how to replace them when you learn how to solve FOR them. Some problems don't give you the value of the variable, for example, writing equations.

In conclusion: It's preparing you for the future when you don't get the variables.

Hope this helps.(1 vote)

## Video transcript

Evaluate the expression
5y to the fourth minus y squared when y is equal to 3. So every place we
see a y here, we could just replace it
with a 3 to evaluate it. So it becomes 5 times 3 to the
fourth power minus 3 squared. All I did is every time we
saw a y here, I put a 3 there. Every time we saw
a y, I put a 3. So what does this evaluate to? And we have to remember
our order of operations. Remember, parentheses
comes first. Sometimes it's
referred to as PEMDAS. Let me write that down. PEMDAS, PEMDAS. P is for parentheses. E is for exponents. M and D are for
Multiplication and Division. They're really at the
same level of priority. And then addition
and subtraction are at the same level. If you really want to do it
properly, it should be P-E, and then multiplication
and division are really at the same level. And addition and subtraction
are at the same level. But what this tells us is
that we do parentheses first. But then after
that, exponentiation takes priority over
everything else here. So we have to evaluate
these exponents before we multiply anything or
before we subtract anything. So the one exponent we'd have
to evaluate is 3 squared. So let's remember. 3 to the first is just 3. It's just 3 times itself once. So it's just 3. 3 squared is equal to 3 times
3, 3 multiplied by itself twice. That's equal to 9. 3 to the third power is
equal to 3 times 3 times 3. Or you could view it
as 3 squared times 3. So it'll be 9. 3 times 3 is 9. 9 times 3 is equal to 27. 3 to the fourth is equal to
3 times 3 times 3 times 3. So 3 times 3 is 9. 3 times 3 is 9. So it's going to be the
same thing as 9 times 9. So this is going
to be equal to 81. So we now know what
3 to the fourth is. We know what 3 squared is. Let's just put it
in the expression. So this is going to be equal
to 5 times 3 to the fourth. 3 to the fourth is 81. So 5 times 81 minus 3 squared. And we have 3 squared
right over here. It is equal to 9. 5 times 81 minus 9. Let's figure out
what 5 times 81 is. So 81 times 5. 1 times 5 is 5. 8 times 5 is 40. So this right over here is 405. So it becomes 405 minus 9. So that is going to be equal
to-- if we were subtracting 10, it would be 395. But we're subtracting
one less than that. So it's 396. And we're done.