Problem 17 - College Algebra

Question

In Exercises 9-30, use the Binomial Theorem to expand each binomial and express the result in simplified form. $$\left(x^{2}+2 y\right)^{4}$$

Step-by-Step Solution

Answer

The binomial \( (x^{2} + 2y)^{4} \) expands into \( x^{8} + 8x^{6}y + 24x^{4}y^{2} + 32x^{2}y^{3} + 16y^{4} \).

Step 1: Understanding the Binomial Theorem

The Binomial theorem states that: \( (a + b)^n = \sum_{k=0}^{n} {n \choose k} a^{n-k}b^{k} \), where \( {n \choose k} \) are the binomial coefficients, also known as the 'n choose k', stating we choose k elements from n.

Step 2: Apply the Binomial Theorem to the Term

Given the binomial \( (x^{2} + 2y)^{4} \), let's proceed to apply the Binomial Theorem to it in the following way: \n\( (x^{2} + 2y)^{4} \) will be expanded as \n\( {4 \choose 0}(x^{2})^{4}(2y)^{0} + {4 \choose 1}(x^{2})^{3}(2y)^{1} + {4 \choose 2}(x^{2})^{2}(2y)^{2} + {4 \choose 3}(x^{2})^{1}(2y)^{3} + {4 \choose 4}(x^{2})^{0}(2y)^{4} \).

Step 3: Simplifying the Expansion

Now, we simplify this expansion by using the binomial coefficient. We also apply the properties of exponents where \( a^{m \times n} = (a^m)^n \). The expanded term simplified will be: \n\( x^{8} + 8x^{6}y + 24x^{4}y^{2} + 32x^{2}y^{3} + 16y^{4} \).

Key Concepts

Binomial Coefficients

When we talk about binomial expansion, binomial coefficients are at the heart of the process. These special numbers appear in the expansion of a binomial expression, represented as \( {n \choose k} \), where 'n' is the exponent and 'k' is the term's position in the expansion.

As seen in our example, when expanding \((x^{2}+2y)^{4}\), we use the binomial coefficients like \({4 \choose 0}\), \({4 \choose 1}\), and so forth. In essence, these coefficients tell us how many ways we can choose 'k' items out of 'n' possibilities. For practical use, they can be calculated using the formula \({n \choose k} = \frac{n!}{k!(n-k)!}\), where '!' denotes factorial, meaning the product of all positive integers up to that number.

These coefficients are symmetric, which means \({n \choose k} = {n \choose n-k}\), and this property makes it easier to determine coefficients, especially for large 'n' values because you can work from both ends of the expansion.

Simplified Form Binomial Expansion

A binomial expansion can initially look complex, but using the Binomial Theorem helps us to express it in a much simpler form. Our example starts with \((x^{2}+2y)^{4}\), which can be daunting to expand manually. By applying the Binomial Theorem, we sequentially increase the power of '2y' while decreasing the power of 'x^2' in each term, making sure that the sum of the exponents is always '4' (the original power).

After placing the binomial coefficients, we get terms like \({4 \choose 0}(x^{2})^{4}(2y)^{0}\), which simplify to just \(x^{8}\). This process is repeated for each term, which simplifies the complex expression to \(x^{8} + 8x^{6}y + 24x^{4}y^{2} + 32x^{2}y^{3} + 16y^{4}\). This simplified form is more manageable and clearly displays how each term relates to its respective binomial coefficient and variable components.

Properties of Exponents

Exponents are subject to a set of rules that make calculations easier, and these properties play a crucial role in simplifying binomial expansions. One primary property used when simplifying the expanded form of our binomial \((x^{2}+2y)^{4}\) is the power of a power rule, which states that \((a^{m})^{n} = a^{m \times n}\), meaning that when you raise a power to another power, you multiply the exponents.

This property helps simplify individual terms after applying the binomial coefficients. For example, we simplified \({4 \choose 1}(x^{2})^{3}(2y)^{1}\) by calculating \(x^{2 \times 3} \times 2y\), which gives us the term \(8x^{6}y\) in the final expansion. Through these properties, which also include product of powers and power of a product, binomial expansion becomes less cumbersome and allows us to convert an expanded polynomial into a series of straightforward terms.