Date | May 2019 | Marks available | 7 | Reference code | 19M.1.SL.TZ1.S_6 |
Level | Standard Level | Paper | Paper 1 | Time zone | Time zone 1 |
Command term | Find | Question number | S_6 | Adapted from | N/A |
Question
The magnitudes of two vectors, u and v, are 4 and √3√3 respectively. The angle between u and v is π6π6.
Let w = u − v. Find the magnitude of w.
Markscheme
* This question is from an exam for a previous syllabus, and may contain minor differences in marking or structure.
METHOD 1 (cosine rule)
diagram including u, v and included angle of π6π6 (M1)
eg
sketch of triangle with w (does not need to be to scale) (A1)
eg
choosing cosine rule (M1)
eg a2+b2−2abcosCa2+b2−2abcosC
correct substitution A1
eg 42+(√3)2−2(4)(√3)cosπ642+(√3)2−2(4)(√3)cosπ6
cosπ6=√32cosπ6=√32 (seen anywhere) (A1)
correct working (A1)
eg 16 + 3 − 12
| w | = √7√7 A1 N2
METHOD 2 (scalar product)
valid approach, in terms of u and v (seen anywhere) (M1)
eg | w |2 = (u − v)•(u − v), | w |2 = u•u − 2u•v + v•v, | w |2 = (u1−v1)2+(u2−v2)2(u1−v1)2+(u2−v2)2,
| w | = √(u1−v1)2+(u2−v2)2+(u3−v3)2√(u1−v1)2+(u2−v2)2+(u3−v3)2
correct value for u•u (seen anywhere) (A1)
eg | u |2 = 16, u•u = 16, u12+u22=16u12+u22=16
correct value for v•v (seen anywhere) (A1)
eg | v |2 = 16, v•v = 3, v12+v22+v32=3v12+v22+v32=3
cos(π6)=√32cos(π6)=√32 (seen anywhere) (A1)
u•v =4×√3×√32=4×√3×√32 (= 6) (seen anywhere) A1
correct substitution into u•u − 2u•v + v•v or u12+u22+v12+v22−2(u1v1+u2v2)u12+u22+v12+v22−2(u1v1+u2v2) (2 or 3 dimensions) (A1)
eg 16 − 2(6) + 3 (= 7)
| w | = √7√7 A1 N2