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Lesson 7: Evaluating expressions word problems

# 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
• 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!
• 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
• 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.
• At , Sal replaces the Y into the equation. What is the logic behind NOT taking all of 5y to the fourth?
• 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.
• PEMDAS can be remembered as
Exponents 2* Excuse
Multiplication x My
Division / Dear
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 !
• 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.
• 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?
• 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.
• 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.
• thank you so much i got i got \$216 for the diving suit is that correct?
• Khan Academy is so fun you just have to put some effort into it.
• 40+40x0+1 =
I'm doing it like this 40+(40x0)+1 =41 is this correct
• yes, in order to make sure you answer it correctly you follow the simple rule:
P ( )
E exponents
M multiply
D divide
S subtract
solve the problem in that order