Question

If \( y = \frac{ax + \beta}{\gamma x + \delta} \), then \( 2y_1 y_3 = \) ?

• A. \( 2y_2^3 \)
• B. \( 3y_2^2 \)
• C. \( y_2^2 \)
• D. \( 3y_3^2 \)

Hint:

For rational functions \( y = \frac{ax + \beta}{\gamma x + \delta} \), use the quotient rule for derivatives and recognize patterns in higher-order derivatives.

Answer 1

The Correct Option is B

Given: \[ y = \frac{ax + \beta}{\gamma x + \delta}. \] Step 1: Finding the first derivative \( y_1 \)
Using the quotient rule: \[ y' = \frac{(\gamma x + \delta)(a) - (ax + \beta)(\gamma)}{(\gamma x + \delta)^2}. \] \[ y' = \frac{a\gamma x + a\delta - a\gamma x - \beta\gamma}{(\gamma x + \delta)^2}. \] \[ y' = \frac{a\delta - \beta\gamma}{(\gamma x + \delta)^2}. \] Step 2: Finding the second derivative \( y_2 \)
Differentiating again: \[ y'' = \frac{d}{dx} \left( \frac{a\delta - \beta\gamma}{(\gamma x + \delta)^2} \right). \] Using the chain rule: \[ y'' = \frac{(a\delta - \beta\gamma)(-2\gamma)}{(\gamma x + \delta)^3}. \] \[ y'' = \frac{-2\gamma (a\delta - \beta\gamma)}{(\gamma x + \delta)^3}. \] Step 3: Finding the third derivative \( y_3 \)
Differentiating again: \[ y''' = \frac{d}{dx} \left( \frac{-2\gamma (a\delta - \beta\gamma)}{(\gamma x + \delta)^3} \right). \] \[ y''' = \frac{-2\gamma (a\delta - \beta\gamma)(-3\gamma)}{(\gamma x + \delta)^4}. \] \[ y''' = \frac{6\gamma^2 (a\delta - \beta\gamma)}{(\gamma x + \delta)^4}. \] Step 4: Computing \( 2y_1 y_3 \)
\[ 2y_1 y_3 = 2 \times \frac{a\delta - \beta\gamma}{(\gamma x + \delta)^2} \times \frac{6\gamma^2 (a\delta - \beta\gamma)}{(\gamma x + \delta)^4}. \] \[ = \frac{12 \gamma^2 (a\delta - \beta\gamma)^2}{(\gamma x + \delta)^6}. \] Since \( y_2^2 = \frac{4\gamma^2 (a\delta - \beta\gamma)^2}{(\gamma x + \delta)^6} \), we get: \[ 2y_1 y_3 = 3y_2^2. \] Thus, the correct answer is: \[ 3y_2^2. \]