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Mathematics: Post your doubts here!

Rutzaba

Rutzaba

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Talha Khatri

Talha Khatri

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Namehere said:
Good! I´m not alone anymore! :D

Someone shine some light on us please! :p
Click to expand...
Guys, just remember, you should always, always find the cross product this way: AB x BC always from one point to another and then from the same common point to the next. Just stick to one thing and don't confuse yourself too much :LOL:
 
Rutzaba

Rutzaba

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daredevil said:
can u explain part ii plzz??? :`)
Click to expand...
the first thingit says is that on q lies theline AB
that means a straight line on ab wud be perpendicular to its normal vector...
that means the directional vector of line= b1 wud be perp to norm vector of plane
so dot product of b1(OB-OA) =(3,1,-1) . (1,b, c) (coeff of x y and z in the plane) = 0
from here we get b=c-3
get this much/
consider this eq 1
then we know the angle between two planes= n1.n2=|n1||n2|cos theta
here they gave us theta to be 60degrees
note that the plane give has norm vector (coeff of eq of plane 1)
(1,1,0)
and (1, b, c)
|n1|= root 2
|n2|= root 1+b^2 +c^2
(1,1,0).(1, b, c)= root 2 * root 1+b^2 +c^2 * -0.5(since acute =60 obtuse 120)
-2(1+b)=root 2 +2^2 +2c^2
substitute the value of c=b-3 into this eq and wala
 
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A

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Rutzaba said:
the first thingit says is that on q lies theline AB
that means a straight line on ab wud be perpendicular to its normal vector...
that means the directional vector of line= b1 wud be perp to norm vector of plane
so dot product of b1(OB-OA) =(3,1,-1) . (1,b, c) (coeff of x y and z in the plane) = 0
from here we get b=c-3
get this much/
Click to expand...
in englis pls :3
 
Serenia

Serenia

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Hello!

I have a doubt in question 8(ii) on this P1 exam.
I could find the value of r when A has a stationary value, but I can't show that there are no straight sections in the track. I looked at the mark scheme but I don't understand that part. The second part (determine the nature of the stationary point) I can do.

Attached I am sending the paper and the mark scheme.

Thanks in advance.
 

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Namehere

Namehere

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Talha Khatri said:
Guys, just remember, you should always, always find the cross product this way: AB x BC always from one point to another and then from the same common point to the next. Just stick to one thing and don't confuse yourself too much :LOL:
Click to expand...

Well, as i mentioned before, I also did it with CA x CB (and many other combinations) and got it right, so your theory fall to pieces there ^^ I just don´t get why using AC x AB or CA x BA I get the wrong answer! :(
 
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