We need to find the ratio of the term in $x^5$ to the term in $x^3$ in the binomial expansion of $(2x + 3)^6$.

AlgebraBinomial TheoremPolynomial ExpansionCoefficientsRatio
2025/4/10

1. Problem Description

We need to find the ratio of the term in x5x^5 to the term in x3x^3 in the binomial expansion of (2x+3)6(2x + 3)^6.

2. Solution Steps

The general term in the binomial expansion of (a+b)n(a+b)^n is given by Tr+1=(nr)anrbrT_{r+1} = \binom{n}{r} a^{n-r} b^r.
For the term with x5x^5 in the expansion of (2x+3)6(2x+3)^6, we need nr=5n-r = 5, so 6r=56-r=5, which means r=1r=1.
So the term with x5x^5 is T1+1=T2=(61)(2x)5(3)1=632x53=576x5T_{1+1} = T_2 = \binom{6}{1} (2x)^5 (3)^1 = 6 \cdot 32x^5 \cdot 3 = 576x^5.
For the term with x3x^3 in the expansion of (2x+3)6(2x+3)^6, we need nr=3n-r = 3, so 6r=36-r=3, which means r=3r=3.
So the term with x3x^3 is T3+1=T4=(63)(2x)3(3)3=6543218x327=208x327=4320x3T_{3+1} = T_4 = \binom{6}{3} (2x)^3 (3)^3 = \frac{6 \cdot 5 \cdot 4}{3 \cdot 2 \cdot 1} \cdot 8x^3 \cdot 27 = 20 \cdot 8x^3 \cdot 27 = 4320x^3.
The ratio of the term in x5x^5 to the term in x3x^3 is 576x54320x3\frac{576x^5}{4320x^3}. However, the problem asks for the *ratio of the terms*, which implies that we are interested in comparing the coefficients, so we compute 5764320\frac{576}{4320}.
5764320=576÷1444320÷144=430=215\frac{576}{4320} = \frac{576 \div 144}{4320 \div 144} = \frac{4}{30} = \frac{2}{15}.
Thus the ratio is 2x215\frac{2x^2}{15}.

3. Final Answer

B. 2x215\frac{2x^2}{15}

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