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Straight Lines and Pair of Straight Lines question

2024 · 27 Jan · Shift 2 · Q56
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Straight Lines and Pair of Straight Lines question

2024 · 27 Jan · Shift 2 · Q56

JEE MainMathematicsStraight Lines and Pair of Straight LinesNumerical+4 / −1
If the sum of squares of all real values of α\alphaα, for which the lines 2x−y+3=0,6x+3y+1=02 x-y+3=0,6 x+3 y+1=02x−y+3=0,6x+3y+1=0 and αx+2y−2=0\alpha x+2 y-2=0αx+2y−2=0 do not form a triangle is ppp, then the greatest integer less than or equal to ppp is ‾\underline{\hspace{2cm}}​.
Numerical answer
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Correct answer: 32

  1. For three lines to not form a triangle, at least one of the following must happen:

    • some two lines are parallel, or
    • all three lines are concurrent.

    Let the lines be: L1:2x−y+3=0L_1: 2x-y+3=0L1​:2x−y+3=0 L2:6x+3y+1=0L_2: 6x+3y+1=0L2​:6x+3y+1=0 L3:αx+2y−2=0L_3: \alpha x+2y-2=0L3​:αx+2y−2=0

  2. First, check whether L1L_1L1​ and L2L_2L2​ are parallel.

    Write in slope form: L1:y=2x+3⇒slope=2L_1: y=2x+3 \quad \Rightarrow \text{slope}=2L1​:y=2x+3⇒slope=2 L2:3y=−6x−1⇒y=−2x−13⇒slope=−2L_2: 3y=-6x-1 \Rightarrow y=-2x-\frac13 \quad \Rightarrow \text{slope}=-2L2​:3y=−6x−1⇒y=−2x−31​⇒slope=−2

    So L1L_1L1​ and L2L_2L2​ are not parallel.

  3. Find values of α\alphaα such that L3L_3L3​ is parallel to L1L_1L1​.

    For L3L_3L3​: 2y=−αx+2⇒y=−α2x+12y=-\alpha x+2 \Rightarrow y=-\frac{\alpha}{2}x+12y=−αx+2⇒y=−2α​x+1 So slope of L3L_3L3​ is −α2-\frac{\alpha}{2}−2α​.

    Parallel to L1L_1L1​ means: −α2=2-\frac{\alpha}{2}=2−2α​=2 α=−4\alpha=-4α=−4

  4. Find values of α\alphaα such that L3L_3L3​ is parallel to L2L_2L2​.

    Parallel to L2L_2L2​ means: −α2=−2-\frac{\alpha}{2}=-2−2α​=−2 α=4\alpha=4α=4

  5. Now check concurrency of all three lines.

    First find intersection point of L1L_1L1​ and L2L_2L2​.

    From L1L_1L1​: y=2x+3y=2x+3y=2x+3

    Substitute into L2L_2L2​: 6x+3(2x+3)+1=06x+3(2x+3)+1=06x+3(2x+3)+1=0 6x+6x+9+1=06x+6x+9+1=06x+6x+9+1=0 12x+10=012x+10=012x+10=0 x=−56x=-\frac56x=−65​

    Then y=2(−56)+3=−53+3=43y=2\left(-\frac56\right)+3=-\frac53+3=\frac43y=2(−65​)+3=−35​+3=34​

    So intersection point is (−56,43)\left(-\frac56,\frac43\right)(−65​,34​)

  6. For concurrency, this point must satisfy L3L_3L3​: αx+2y−2=0\alpha x+2y-2=0αx+2y−2=0 α(−56)+2(43)−2=0\alpha\left(-\frac56\right)+2\left(\frac43\right)-2=0α(−65​)+2(34​)−2=0 −5α6+83−2=0-\frac{5\alpha}{6}+\frac83-2=0−65α​+38​−2=0 −5α6+23=0-\frac{5\alpha}{6}+\frac23=0−65α​+32​=0 −5α+4=0-5\alpha+4=0−5α+4=0 α=45\alpha=\frac45α=54​

  7. Therefore all real values of α\alphaα for which the three lines do not form a triangle are: α=−4, 4, 45\alpha=-4,\ 4,\ \frac45α=−4, 4, 54​

  8. Sum of squares of these values: p=(−4)2+42+(45)2p=(-4)^2+4^2+\left(\frac45\right)^2p=(−4)2+42+(54​)2 p=16+16+1625p=16+16+\frac{16}{25}p=16+16+2516​ p=32+1625=81625=32.64p=32+\frac{16}{25}=\frac{816}{25}=32.64p=32+2516​=25816​=32.64

  9. Hence, ⌊p⌋=⌊32.64⌋=32\lfloor p \rfloor = \lfloor 32.64 \rfloor = 32⌊p⌋=⌊32.64⌋=32

So the required integer is 323232.

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