SOLUTION: Rewrite the expression x^3-2x^2y+xy^2 so that it is easy to calculate the its value without a calculator when x=21 and y=19.

Question 132850: Rewrite the expression x^3-2x^2y+xy^2 so that it is easy to calculate the its value without a calculator when x=21 and y=19.
Answer by jim_thompson5910(35256) (Show Source): You can put this solution on YOUR website!
If you factor this expression, then it will be much easier to calculate the result

Start with the given expression

Factor out the GCF

Now let's focus on the inner expression

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Looking at we can see that the first term is and the last term is where the coefficients are 1 and 1 respectively.

Now multiply the first coefficient 1 and the last coefficient 1 to get 1. Now what two numbers multiply to 1 and add to the middle coefficient -2? Let's list all of the factors of 1:

Factors of 1:
1

-1 ...List the negative factors as well. This will allow us to find all possible combinations

These factors pair up and multiply to 1
1*1
(-1)*(-1)

note: remember two negative numbers multiplied together make a positive number

Now which of these pairs add to -2? Lets make a table of all of the pairs of factors we multiplied and see which two numbers add to -2

First Number	Second Number	Sum
1	1	1+1=2
-1	-1	-1+(-1)=-2

From this list we can see that -1 and -1 add up to -2 and multiply to 1

Now looking at the expression , replace with (notice adds up to . So it is equivalent to )

Now let's factor by grouping:

Group like terms

Factor out the GCF of out of the first group. Factor out the GCF of out of the second group

Since we have a common term of , we can combine like terms

So factors to

So this also means that factors to (since is equivalent to )

note: is equivalent to since the term occurs twice. So also factors to

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So our expression goes from and factors further to

So factors to

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Now let's evaluate the expression when x=21 and y=19

Now plug in x=21 and y=19

Subtract

Square 2 to get 4

Multiply

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Answer:

So when x=21 and y=19
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