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Straight Lines and Pair of Straight Lines question

2024 · 1 Feb · Shift 2 · Q60
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Straight Lines and Pair of Straight Lines question

2024 · 1 Feb · Shift 2 · Q60

JEE MainMathematicsStraight Lines and Pair of Straight LinesNumerical+4 / −1
The lines L1, L2,…,L20\mathrm{L}_1, \mathrm{~L}_2, \ldots, \mathrm{L}_{20}L1​, L2​,…,L20​ are distinct. For n=1,2,3,…,10\mathrm{n}=1,2,3, \ldots, 10n=1,2,3,…,10 all the lines L2n−1\mathrm{L}_{2 \mathrm{n}-1}L2n−1​ are parallel to each other and all the lines L2nL_{2 n}L2n​ pass through a given point PPP. The maximum number of points of intersection of pairs of lines from the set {L1, L2,…,L20}\left\{\mathrm{L}_1, \mathrm{~L}_2, \ldots, \mathrm{L}_{20}\right\}{L1​, L2​,…,L20​} is equal to ‾\underline{\hspace{2cm}}​.
Numerical answer
View written solutionFree

Correct answer: 101

  1. Classify the 20 lines

There are two groups:

  • Odd-indexed lines: L1,L3,…,L19L_1, L_3, \dots, L_{19}L1​,L3​,…,L19​

    These are 101010 distinct lines, all parallel to each other.

  • Even-indexed lines: L2,L4,…,L20L_2, L_4, \dots, L_{20}L2​,L4​,…,L20​

    These are 101010 distinct lines, all passing through a fixed point PPP.

We want the maximum number of distinct intersection points of pairs of lines.


  1. Intersections among odd-indexed lines

Since all 101010 odd-indexed lines are parallel, no two of them intersect.

So, number of intersection points from this group: 000


  1. Intersections among even-indexed lines

All 101010 even-indexed lines pass through the same point PPP.

Any pair of these lines intersects at PPP, but this gives only one distinct intersection point.

So, number of distinct intersection points from this group: 111


  1. Intersections between an odd-indexed line and an even-indexed line

Each odd-indexed line is parallel to the other odd-indexed lines, and each even-indexed line passes through PPP.

To maximize the number of distinct intersection points:

  • none of the odd-indexed lines should pass through PPP,
  • no even-indexed line should be parallel to the odd family.

Then each odd line intersects each even line in exactly one point.

Number of such pairs: 10×10=10010 \times 10 = 10010×10=100

Now check whether these 100100100 intersection points are distinct.

  • For a fixed even line, its intersections with the 101010 parallel odd lines are distinct.
  • For two different even lines, their intersection with a given odd line are different because the even lines meet only at PPP, and we ensured the odd lines do not pass through PPP.

Hence all these 100100100 points can be made distinct.

So, number of distinct intersection points from mixed pairs: 100100100


  1. Total maximum number of distinct intersection points

Adding all contributions: 0+1+100=1010 + 1 + 100 = 1010+1+100=101


  1. Final Answer

The maximum number of points of intersection is 101\boxed{101}101​

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