Math Problem Statement

Find the coefficient of x^7 in the expansion of (2x-3)^7 Pick the correct answer -2187, -128, 2, or 128

Solution

To find the coefficient of x7x^7 in the expansion of (2x3)7(2x - 3)^7, we can use the Binomial Theorem. The Binomial Theorem states:

(a+b)n=k=0n(nk)ankbk(a + b)^n = \sum_{k=0}^{n} \binom{n}{k} a^{n-k} b^k

In this case, a=2xa = 2x, b=3b = -3, and n=7n = 7.

We want the term where the power of xx is exactly 7. In the expansion, the general term looks like this:

Tk=(7k)(2x)7k(3)kT_k = \binom{7}{k} (2x)^{7-k} (-3)^k

We need the power of xx to be 7, which means the exponent of xx in (2x)7k(2x)^{7-k} must be 7. This occurs when 7k=77 - k = 7, or k=0k = 0.

Now, let's calculate the term when k=0k = 0:

T0=(70)(2x)7(3)0T_0 = \binom{7}{0} (2x)^7 (-3)^0

This simplifies to:

T0=1(2x)71=(2x)7=128x7T_0 = 1 \cdot (2x)^7 \cdot 1 = (2x)^7 = 128x^7

Thus, the coefficient of x7x^7 is 128128.

The correct answer is 128.

Would you like more details on this process, or do you have any questions?

Here are 5 related questions for further exploration:

  1. How would the coefficient change if the exponent were higher, say (2x3)10(2x - 3)^{10}?
  2. What if the signs inside the binomial were different, such as (2x+3)7(2x + 3)^7?
  3. How can the Binomial Theorem be applied to expansions with fractional exponents?
  4. How would you expand (2x3)7(2x - 3)^7 without using the Binomial Theorem?
  5. What is the general form of the Binomial Theorem for negative exponents?

Tip: Always double-check the signs in binomial expansions, especially when terms involve subtraction!

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Math Problem Analysis

Mathematical Concepts

Algebra
Binomial Expansion
Exponents

Formulas

(a + b)^n = Σ (n choose k) * a^(n-k) * b^k

Theorems

Binomial Theorem

Suitable Grade Level

Grades 9-12