Solve this for me please!!

In a plane there are 37 straight lines of which 13 pass through point A A and 11 pass through B B . Besides no three lines pass through one point, no line passes through both points A A and B B , and no two are parallel. Then find number of intersection points the lines have.


Answer will be posted in two days if no one else posts it!!


The answer is 535.

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

3 solutions

Qi Huan Tan
Jan 28, 2015

We have ( 37 2 ) 37 \choose 2 ways to choose a pair of lines from the 37 points. Each pair will determine a unique intersection point except among those 13 points passed through point A A and among those 11 points passed through point B B . Hence, the number of intersection points is ( 37 2 ) ( 13 2 ) + 1 ( 11 2 ) + 1 = 535 {37 \choose 2} - {13 \choose 2} + 1 - {11 \choose 2} + 1 = 535 .

Nihar Mahajan
Dec 19, 2014

Let each line that passes through point A be known as an 'A' line. Let each line that passes through point B be known as a 'B' line. Let each line that passes through neither point A nor point B be known as an 'K' line.

Since there 13 A lines, 11 B lines, and a total of 37 lines, the number of K lines = 37-13-11 = 13.

Case 1: An A line intersects with a B line Number of options for the A line = 13. Number of options for the B line = 11. To combine these options, we multiply: 13*11 = 143.

Case 2: An N line intersects with an A or B line Number options for the N line = 13. Number of options for the A or B line = 13+11 = 24. To combine these options, we multiply: 13*24 = 312.

Case 3: An N line intersects with another N line Each PAIR of N lines will yield an intersection. From the 13 N lines, the number of ways to choose 2 = 13C2 = (13 * 12) / ( 2 * 1 ) = 78.

Case 4: Points A and B Points A and B constitute 2 more intersections.

Total intersections = 143+312+78+2 = 535.

Gustavo Jambersi
Nov 3, 2015

A = 13.12÷2=78 pairs of lines B = 11.10÷2=55 pairs of lines

Number of intersactions=37.36÷2=666 But we are counting 78 times the point A and 55 times the point B

So: 666 - 78 - 55= 533

But now we don't count A and B

Finally : 533 + 2 = 535

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...