Here's the question you clicked on:
nexis
Linear Algebra: Question I-15: http://ocw.mit.edu/courses/mathematics/18-02sc-multivariable-calculus-fall-2010/part-a-vectors-determinants-and-planes/problem-set-1/MIT18_02SC_SupProb1.pdf Answer at: http://ocw.mit.edu/courses/mathematics/18-02sc-multivariable-calculus-fall-2010/part-a-vectors-determinants-and-planes/problem-set-1/MIT18_02SC_SupProbSol1.pdf In this question, how do we find dir QP?
I think you mean the question 1I-5. If you know the angle between a vector and x-axis, then you can get the vector's direction. In this question, draw a horizontal line L through point Q, the angle between QP and L is θ-π/2, then we can find dir QP=<cos(θ-π/2),sin(θ-π/2)>=....