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TS EAMCET · Maths · Binomial Theorem

If \((a+b x)^{-3}=\frac{1}{27}+\frac{1}{3} x+\ldots\), then the ordered pair \((a, b)\) equals to

  1. A \((3,-27)\)
  2. B \(\left(1, \frac{1}{3}\right)\)
  3. C \((3,9)\)
  4. D \((3,-9)\)
Verified Solution

Answer & Solution

Correct Answer

(D) \((3,-9)\)

Step-by-step Solution

Detailed explanation

\begin{aligned} \text { Now, }(a+b x)^{-3} & =a^{-3}\left(1+\frac{b x}{a}\right)^{-3} \\ & =\frac{1}{a^3}\left(1-{ }^3 C_1\left(\frac{b x}{a}\right)+\ldots\right) \\ \Rightarrow \quad \frac{1}{a^3} & -{ }^3 C_1 \frac{1}{a^3}\left(\frac{b x}{a}\right)+\ldots \\ &…