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Question: Answered & Verified by Expert
If the ratio of the roots of the equation \(\mathrm{px}^2+\mathrm{qx}+\mathrm{r}=0\) is \(a: b\), then \(\frac{\mathrm{ab}}{(\mathrm{a}+\mathrm{b})^2}=\)
MathematicsQuadratic EquationWBJEEWBJEE 2011
Options:
  • A \(\frac{\mathrm{p}^2}{\mathrm{qr}}\)
  • B \(\frac{\mathrm{pr}}{\mathrm{q}^2}\)
  • C \(\frac{q^2}{\mathrm{pr}}\)
  • D \(\frac{\mathrm{pq}}{\mathrm{r}^2}\)
Solution:
1353 Upvotes Verified Answer
The correct answer is: \(\frac{\mathrm{pr}}{\mathrm{q}^2}\)
Hints: Let roots are \(a \alpha\) and \(b \alpha\)
\(\Rightarrow(\mathrm{a}+\mathrm{b}) \alpha=\frac{-\mathrm{q}}{\mathrm{p}}\)
\(\begin{aligned} & a b \alpha^2=\frac{r}{p} \\ & \frac{a b \alpha^2}{(a+b)^2 \alpha^2}=\frac{r}{p} \cdot \frac{p^2}{q^2} \\ & \frac{a b}{(a+b)^2}=\frac{1 p}{q^2}\end{aligned}\)

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