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MHT CET · Maths · Pair of Lines

If the slope of one of the lines represented by \(a x^2+(2 a+1) x y+2 y^2=0\) is reciprocal of the slope of the other, then the sum of squares of slopes is

  1. A \(\frac{17}{4}\)
  2. B \(\frac{82}{9}\)
  3. C \(\frac{97}{36}\)
  4. D 2
Verified Solution

Answer & Solution

Correct Answer

(A) \(\frac{17}{4}\)

Step-by-step Solution

Detailed explanation

Given equation of pair of lines is
\(\begin{aligned}
& \mathrm{a} x^2+(2 \mathrm{a}+1) x y+2 y^2=0 \\
& \mathrm{~A}=\mathrm{a}, \mathrm{H}=\frac{2 \mathrm{a}+1}{2}, \mathrm{~B}=2
\end{aligned}\)
Given condition,
\(\mathrm{m}_1=\frac{1}{\mathrm{~m}_2} \)
\( \therefore \mathrm{m}_1 \cdot \mathrm{m}_2=1 \)
\( \text { Product of slopes }=\frac{\mathrm{A}}{\mathrm{B}}=\frac{\mathrm{a}}{2} \)
\( \therefore \mathrm{m}_1 \cdot \mathrm{m}_2=1=\frac{\mathrm{a}}{2} \)
\( \therefore \mathrm{a}=2 \)
\( \text { Also, sum of slopes }=\frac{-2 \mathrm{H}}{\mathrm{B}}=-\left(\frac{2 \mathrm{a}+1}{2}\right)=\frac{-5}{2} \)
\( \text { Using }\left(\mathrm{m}_1+\mathrm{m}_2\right)^2=\mathrm{m}_1^2+\mathrm{m}_2^2+2 \mathrm{~m}_1 \mathrm{~m}_2 \)
\( \left(\frac{-5}{2}\right)^2=\mathrm{m}_1^2+\mathrm{m}_2^2+2 \times 1 \)
\( \therefore \mathrm{m}_1^2+\mathrm{m}_2^2=\frac{25}{4}-2 \)
\( \therefore \mathrm{m}_1^2+\mathrm{m}_2^2=\frac{17}{4}\)