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MATH1002 Linear Algebra
Answers are provided below. Important Ideas and Useful Facts: (i) In R3, the cross product of u = [u1, u2, u3] and v = [v1, v2, v3] is the vector u× v = [u2v3 − u3v2, u3v1 − u1v3, u1v2 − u2v1]. (ii) The cross product u× v is orthogonal to both u and v. (iii) The direction of u× v is given by the right-hand rule. (iv) Anti-commutativity of cross product: For all vectors u and v in R3, u× v = −(v × u). (v) Distributivity of cross product: For all vectors u, v and w in R3, (u+ v)×w = u×w + v ×w. (vi) If v and w are vectors in R3 and c is a scalar then (cv)×w = c(v ×w) = v × (cw). (vii) For all vectors u in R3, u× u = 0. (viii) If θ is the angle between vectors v and w in R3 then ‖v ×w‖ = ‖v‖‖w‖ sin θ. (ix) The area of the parallelogram inscribed by v and w in R3 is ‖v × w‖, and the area of the triangle inscribed by v and w is 1 2 ‖v ×w‖. (x) A normal vector for a line ℓ is a nonzero vector n which is orthogonal to any vector parallel to ℓ. (xi) A direction vector for a line ℓ is a nonzero vector d which is parallel to ℓ. If A and B are distinct points on ℓ then −−→ AB is a direction vector for ℓ. (xii) The normal form of the equation for a line ℓ in R2 is n · (x− p) = 0, or, equivalently, n · x = n · p, where p is a point on ℓ, and n is a normal vector for ℓ. (xiii) The general form of the equation for a line ℓ in R2 is ax+ by = c, where n = [a, b] is a normal vector for ℓ. (xiv) The vector form of the equation for a line ℓ in R2 or R3 is x = p+ td, where p is a point on ℓ, d is a direction vector for ℓ and t ∈ R. (xv) The parametric equations of ℓ are the equations corresponding to the components: x = p1 + td1 and y = p2 + td2 (and, for lines in R 3, z = p3 + td3), for t ∈ R. 1 Preparatory Exercises: 1. Given that a = [2,−1, 2],b = [1, 1,−1] and c = [3, 0,−4], find (i) a× b (ii) the angle between a and a× b (iii) ‖a× b‖ (iv) the sine of the angle between a and b. 2. Let ℓ be the line in R3 passing through the point (2, 3, 5) in the direction of [1, 3,−1]. Find a vector form for ℓ and parametric equations for ℓ. Answers: 1. (i) [−1, 4, 3] (ii) pi 2 (iii) √ 26 (iv) √ 78 9 2. x = [2, 3, 5] + t[1, 3,−1] where t ∈ R, x = 2 + t y = 3 + 3t z = 5− t t ∈ R.