Substituting Values into Algebraic Expressions

[faith21]

If x = –1 and y = 2, what is the value of the expression 2x3 – 3xy ?

okay i know this its suppose to be easy but my answer does not match the book what am i doing wrong

i did:
2x^3-3xy
2(-1^3)-3(-1+2)

and i did p-e-m-d-a-s

2(-1)-3(3)

and i got 2-9 which is 7

so are they saying that the answer is 4 where did they get four from

 

[mmm4444bot]

2x^3-3xy

2(-1^3)-3(-1+2)

Firstly, we need to be careful when we type negative one cubed.

(-1)^3

Secondly, xy is x times y. You added y to x, instead.

With corrections, we have the following.

2(-1)^3 - 3(-1)(2)

Now continue with the Order of Operations.

Be careful. I see you wrote 2(-1) as 2. (You forgot to multiply them.)

 

[faith21]

Re:

2x^3-3xy

2(-1^3)-3(-1+2)

Firstly, we need to be careful when we type negative one cubed.

(-1)^3

Secondly, xy is x times y. You added y to x, instead.

With corrections, we have the following.

2(-1)^3 - 3(-1)(2)

Now continue with the Order of Operations.

Be careful. I see you wrote 2(-1) as 2. (You forgot to multiply them.)

am i suppose to use P-E-M-D-A-S cuz i just did and i still didnt get four i dont mean to sound dumb am just still a lil confused.

cuz i did:
2(-1)3^-3(-1)(2)
2(3)-3(-2)
and yea judging by tht i get 0 lol so plz plz plz pretend am 6 am terrible with math

 

[faith21]

in algebra what does a equal sign with a slash through it mean

 

[mmm4444bot]

2(-1)3^-3(-1)(2)

2(3)-3(-2)

PEMDAS:

Parentheses -- There is nothing to do within any of the three sets of parentheses because they each already contain a single number.

Exponents -- There is only one exponentiation to do: (-1)^3

Do you know? (-1)^3 = (-1) * (-1) * (-1)

What do we get when we multiply three factors of negative one together?

We do not get 3, as you typed. Please try again, and fill in the following blank.

2( _ ) - 3(-2)

Multiplication -- There are two multiplications to do, working from left to right.

The first is: two times ( _ ).

The second is: negative three times negative two.

Do you see why?

Division -- There are no divisions in this exercise.

Addition -- There is one addition to do. Add the two numbers that you get from each multiplication above.

Subtraction -- There are none.

There are none because I interpreted - 3(-2) as + (-3)(-2).

In other words, instead of multiplying -2 by +3 and subtracting the result from 2( _ ), I did the following "switch" in my head.

2( _ ) - 3(-2)

2( _ ) + (-1)(3)(-2)

2( _ ) + (-3)(-2)

Let me know if you don't understand this "switch". It's common to interpret subtraction as adding the opposite.

MY EDIT: Clarified original statement that there is no subtraction

 

[mmm4444bot]

in algebra what does [an] equal sign with a slash through it mean

It means "not equal to".

When typing it, we use the symbol <> instead.

EG:

1 <> 2

 

[faith21]

Re:

2(-1)3^-3(-1)(2)

2(3)-3(-2)

PEMDAS:

Parentheses -- There is nothing to do within any of the three sets of parentheses because they each already contain a single number.

Exponents -- There is only one exponentiation to do: (-1)^3

Do you know? (-1)^3 = (-1) * (-1) * (-1)

What do we get when we multiply three factors of negative one together?

We do not get 3, as you typed. Please try again, and fill in the following blank.

2( _ ) - 3(-2)

Multiplication -- There are two multiplications to do, working from left to right.

The first is: two times ( _ ).

The second is: negative three times negative two.

Do you see why?

Division -- There are no divisions in this exercise.

Addition -- There is one addition to do. Add the two numbers that you get from each multiplication above.

Subtraction -- There are none.

wow thanks my problem is i wasnt sure i was using the right formula i new what they stud for but i was doin it right i got it now

i did:
2x^3 - 3xy
2(-1)^3 - 3(-1)(2)
2(-1)-3(-1)(2)
2-6=4

is thats right?

 

[mmm4444bot]

… 2x^3 - 3xy

2(-1)^3 - 3(-1)(2)

2(-1) - 3(-1)(2)

2 - 6 = 4

No, but you got the substitutions and the (-1)^3 part correct.

2(-1) means two times negative one.

This does not equal 2; it equals negative 2.

Anytime we multiply a real number by -1, we get the real number's opposite.

2(-1) = -2

(-2)(-1) = 2

4(-1) = -4

x(-1) = -x

(-x)(-1) = x

Do these examples make sense?

So, fixing that mistake gives us the following.

-2 - 3(-1)(2)

The Order of Operations tells us that we need to do the other multiplications before subtracting.

(This time, I will NOT make the "switch" from subtraction to addition, as discussed in my previous post.)

Working from left to right, there are two multiplications. The first is 3(-1).

Multiplying 3 by negative 1 gives us the opposite of 3. This gives us the following.

-2 - (-3)(2)

The final multiplication is (-3)(2).

What do you get when you multiply negative 3 by 2?

Subtract that amount from -2, and you're done.

Please show me what you get.