Maths P3 !!!Need help in finding perpendicular distance between point and line!!!

Discussion in 'International A And AS Level' started by Mayukh, Oct 11, 2015.

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1. Mayukh

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Can someone please help me with vectors. For some reason, whenever I do any of the maths involving finding the distance between a point and a line, the answers never match. Can someone give me a tip on how to solve this type of problems.

When I solve it I find the unit vector in the direction of the line. Let's call it u. I also find the vector of a line from a part of r=a+tb. Let's call it c.

I find c.u, |c| and take the shortest distance as sqrt{(c.u)^2+(|c|)^2}

Could somebody please tell me what I'm doing wrong?
Thanks

3. Serial_Ripper

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Um...I'm not that good at vectors either, but the answers usually match......I can explain in if you give me one of the questions which are bothering you.

4. Mayukh

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One of the questions is q2 from the p3 book.

find the distance of (1,0,0) from r=t(12i-3j-4k)

my soln:
a=i
unit vector in direction r, n=(12/13)i-(3/13)j-(4/13)k
a.n=12/13
|a|=1
so, shortest distance=sqrt(1-(12/13)^2)=5/13

answer on the back of the book is (1/13)*sqrt(69)

5. Serial_Ripper

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SORRY FOR BEING LATE!!
I THINK IT'S BETTER IF YOU USE EXAMPLES FROM PAST PAPERS...
I took screenshots of the way I solve this type of question but i've got no idea how to post the photo...PLZ TELL ME HOW!
DON'T WORRY! THE MAIN PROB WITH THIS QUESTION IS CONCENTRATION !

6. nehaoscar

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Just copy the screenshot and paste it here

7. Serial_Ripper

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Not working

8. nehaoscar

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Try using the snipping tool if you have in the computer ... that works for me when i paste it

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9. Serial_Ripper

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10. Serial_Ripper

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hope you'll find them useful!

11. Serial_Ripper

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THNX!

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