MHB Apply Binomial Theorem: Expand (x-2y)^3

AI Thread Summary
To expand the binomial expression (x - 2y)^3 using the binomial theorem, the formula is applied as (a + b)^n = Σ(n choose r) a^(n-r) b^r. Rewriting (x - 2y) as (x + (-2y)), the expansion results in the sum of terms involving coefficients from the binomial coefficients. The final expanded form is x^3 - 6x^2y + 12xy^2 - 8y^3. This demonstrates the application of the binomial theorem to expand the given expression accurately.
MarkFL
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Here is the question:

How to expand this binomial expansion?


a.) (x - 2y)^3

with the equation:

(n over r) x [a^(n-r)] x (b^r)

Thank you!

I have posted a link there to this topic so the OP can see my work.
 
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Hello Person,

The binomial theorem may be stated as:

$$(a+b)^b=\sum_{r=0}^{n}{n \choose r}a^{n-r}b^r$$

And so, for the given binomial to be expanded, we have:

$$(x-2y)^3=(x+(-2y))^3=\sum_{r=0}^{3}{3 \choose r}x^{n-r}(-2y)^r$$

$$(x-2y)^3={3 \choose 0}x^3(-2y)^0+{3 \choose 1}x^2(-2y)^1+{3 \choose 2}x^1(-2y)^2+{3 \choose 3}x^0(-2y)^3$$

$$(x-2y)^3=1\cdot x^3\cdot1+3x^2(-2y)+3x(-2y)^2+1\cdot1\cdot(-2y)^3$$

$$(x-2y)^3=x^3-6x^2y+12xy^2-8y^3$$
 
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