[Schteve]

Think I got 72/75 in C3 going by Arsey's solutions, so need about 65-68 on this to get an A*... Am bricking it

Good luck everyone!

[Sarah456]

Good luck to everyone. High probability i will screw up =/. I hope they gave simple differential equations - nothing too tricky -_-

[-Illmatic-]

(Original post by Sarah456)
Good luck to everyone. High probability i will screw up =/. I hope they gave simple differential equations - nothing too tricky -_-

Lol exactly what im thinking. Simple vectors as well please

[Sarah456]

(Original post by -Illmatic-)
Lol exactly what im thinking. Simple vectors as well please

A big chance we won't be getting a simple differential equation as the last two years - they were pretty simple. I find vectors comparably alot easier..but yeah, its usually the last part for me, which really kicks me...Damn you C4..-__-and C3...but not as much. =/

[Mmrawrr]

Which solomon papers did you find the hardest? I'll give them a go.

[lekha2611]

Last minute question!
If you have a line, say (1,2,3) + lambda(2,0,1) and you want to find the coordinates of a point on the line, which you know to be perpendicular (from the line to the origin) how would you work out it's coordinates?
I thought you would dot product (x,y,z) and (2,0,1) and then get: 2x+z=0. Then I would sub in random numbers, like x=2, to find z=-4, to get (2,0,-1), but this isn't correct apparently. :-S

[King_Arthur]

do you know the correct answer

[lekha2611]

(Original post by anjelofernando)
do you know the correct answer

Sorry, I missed out that there's another point, A, on the line, (1,2,3).
But then, say you let the point where the line is perp. to the origin be F, then it has coordinates (x,y,z).
Then, if you work out AF (-1+x,-2+y,-3+z) and OF(x,y,z) and dot product, then you have three unknowns- which is more confusing :-S

[alexsasg]

On vector questions where is asks you to find the coordinates of A (the point of intersection - this tends to be a 1 mark question, I think), I always find lambda and the other symbol first, then substitute in to find A. This is really long though (far too long for one mark!) so how else could I find A?

[jhonwds]

(Original post by alexsasg)
On vector questions where is asks you to find the coordinates of A (the point of intersection - this tends to be a 1 mark question, I think), I always find lambda and the other symbol first, then substitute in to find A. This is really long though (far too long for one mark!) so how else could I find A?

Post the question so that we can help you

[King_Arthur]

(Original post by lekha2611)
No sorry, but I'm really just looking for methods tbh

this is my opinion im not that sure if im correct though

the problem is that xyz in this case is positional vector not a directional vector so it cant be used in the dot product , and the data is slightly not enough

[lekha2611]

(Original post by anjelofernando)
this is my opinion im not that sure if im correct though

the problem is that xyz in this case is positional vector not a directional vector so it cant be used in the dot product , and the data is slightly not enough

I know it's a dodgy question, but how else would you find the position vector of a point on the line, given another point and the vector equation of the line itself?

[Sanjay94]

Good luck all

If you see a question that you can't solve immediately, just stay calm and think clearly about what the question is asking you. Remember that all the information you need to answer a question will be written down, or you will have worked it out previously.

[Intriguing Alias]

(Original post by lekha2611)
I know it's a dodgy question, but how else would you find the position vector of a point on the line, given another point and the vector equation of the line itself?

Can you post an example because the way you're describing it isn't making much sense to me?

[lekha2611]

(Original post by hassi94)
Can you post an example because the way you're describing it isn't making much sense to me?

http://dl.dropbox.com/u/36280449/TSR...20question.bmp

[King_Arthur]

(Original post by lekha2611)
Sorry, I missed out that there's another point, A, on the line, (1,2,3).
But then, say you let the point where the line is perp. to the origin be F, then it has coordinates (x,y,z).
Then, if you work out AF (-1+x,-2+y,-3+z) and OF(x,y,z) and dot product, then you have three unknowns- which is more confusing :-S

i got an answer -1,2,2 but in a very long and not sure way

[jhonwds]

(Original post by lekha2611)
http://dl.dropbox.com/u/36280449/TSR...20question.bmp

I used the dot theorem with OF and the line l1 to find when their product is equal to zero.

OF is given by the equation as (1+2λ,2,3+λ) and the direction of the line as (2,0,1)

Then using the dot product

(2)(1+2λ)+(1)(3+λ)=0
2+4λ+3+λ=0
5λ=-5
so λ = -1

[lekha2611]

(Original post by jhonwds)
I used the dot theorem with OF and the line l1 to find when their product is equal to zero.

OF is given by the equation as (1+2λ,2,3+λ) and the direction of the line as (2,0,1)

Then using the dot product

(2)(1+2λ)+(1)(3+λ)=0
2+4λ+3+λ=0
5λ=-5
so λ = -1

Why is OF (1+2λ,2,3+λ)?

[King_Arthur]

(Original post by jhonwds)
I used the dot theorem with OF and the line l1 to find when their product is equal to zero.

OF is given by the equation as (1+2λ,2,3+λ) and the direction of the line as (2,0,1)

Then using the dot product

(2)(1+2λ)+(1)(3+λ)=0
2+4λ+3+λ=0
5λ=-5
so λ = -1

i got lambda as -1

[jhonwds]

(Original post by lekha2611)
Why is OF (1+2λ,2,3+λ)?

The equation of a line in vector form, gives you the position vector of any point that lies on the line.

Taking the line equation we have (1,2,3)+λ(2,0,1) we can say that any point that lies on the line is given by (1+2λ,2+λ(0),3+λ) so (1+2λ,2,3+λ).