OCR (not MEI) FP3 mark scheme error june 08

Fairly sure you use cos for the scalar/dot product? Yet the markscheme uses sin, see question two on the linked question paper and mark scheme.

Thanks in advance.

http://www.ocr.org.uk/Images/59545-question-paper-unit-4727-further-pure-mathematics-3.pdf
http://www.ocr.org.uk/Images/62961-mark-scheme-june.pdf

Reply 1

SerialVelocity

Original post by kabutsu12

The reason it's using sin instead of cos is because the question is asking for the angle between the vector and the plane instead of the angle between the vector and the normal.

(edited 10 years ago)

Reply 2

sj_1995

You can still use the dot product with cos, but just remember to take your angle away from 90. In this context the dot product you are calculating is between the line and the normal vector through the plane.

Therefore, the angle between the line and the plane itself is 90 - (angle calculated above through using cos(theta) = a.b/[a] )

I hope that make sense and helps

Reply 3

kabutsu12

Yeah, awesome. Thats the method I'd use as I find having to do the cross product a little bit cumbersome. If you were to use the cross product method and you got the unit vector n in the numerator, what effect does this have?