Chapter 4: Problem 17
Find the coefficient of \(x^{6}\) in the expansion of \((2 x-3)^{9}\)
Short Answer
Expert verified
The coefficient of \(x^6\) is \(-145152\).
Step by step solution
01
Identify the General Term
The general term in the binomial expansion of \((a + b)^n\) is given by \(T_{k+1} = \binom{n}{k} a^{n-k} b^k\). Here, \(a = 2x\), \(b = -3\), and \(n = 9\). We need to find the term containing \(x^6\).
02
Set up the Term Containing \(x^6\)
We express the general term as follows: \(T_{k+1} = \binom{9}{k} (2x)^{9-k} (-3)^k\). The power of \(x\) is \((2x)^{9-k} = 2^{9-k}x^{9-k}\). For the term containing \(x^6\), set \(9-k = 6\), which gives \(k = 3\).
03
Calculate the Specific Term
Substitute \(k = 3\) into the general term formula: \[T_{4} = \binom{9}{3} (2x)^{6} (-3)^3 = \binom{9}{3} \cdot 2^6 x^6 \cdot (-27)\].
04
Compute the Numerical Coefficient
Calculate each component: \(\binom{9}{3} = 84\), \(2^6 = 64\), and \(-27\). Multiply these together to find the coefficient: \[84 \cdot 64 \cdot (-27) = -145152\].
05
Conclusion
The coefficient of \(x^6\) is \(-145152\). This is the numeric factor that multiplies \(x^6\) in the expansion of \((2x - 3)^9\).
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Coefficient Calculation
When dealing with the coefficient of a specific term in a polynomial expansion, such as finding the coefficient of \(x^6\) in \((2x-3)^9\), it is crucial to understand the framework of the binomial theorem. The binomial theorem is used to expand expressions of the form \((a + b)^n\) into a sum of terms. Each term in this expansion has a coefficient, which is determined by a combination formula.The general term in a binomial expansion is \(T_{k+1} = \binom{n}{k} a^{n-k} b^k\). This formula incorporates:
- \(\binom{n}{k}\) - The binomial coefficient, which shows how many ways \(k\) items can be chosen from \(n\) items, calculated as \(\frac{n!}{k!(n-k)!}\).
- \(a^{n-k}\) and \(b^k\) - The remaining parts of the binomial expression \((a+b)^n\).
Polynomial Expansion
Polynomial expansion involves converting expressions like \((2x-3)^9\) into a sum of individual terms. The Binomial Theorem provides the method for this, stating that \((a + b)^n\) expands into \(\sum_{k=0}^{n} \binom{n}{k} a^{n-k} b^k\). This generates each term by addressing combinations of the elements and their respective powers.Each term in the expansion has:
- An integer coefficient (binomial coefficient)
- A power of the first term, \(a^{n-k}\)
- A power of the second term, \(b^k\)
Exponential Expressions
Exponential expressions are dominant in polynomial expansions like \((2x-3)^9\). Here, each term is influenced by the powers of its components, \(a\) and \(b\), in the binomial expansion.Core to understanding how these terms develop is identifying their exponential nature:
- The term \((2x)^{9-k}\) forms when the base of \(x\) is raised by \(n-k\). This affects both the power of \(x\) and its coefficient.
- Simultaneously, the term \((-3)^k\) arises as the second element, with its influence growing as \(k\) increases.