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Question

If a and b are two non-collinear unit vectors such that a +b =3, find 2a -5b · 3a +b .

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Solution

We havea+b=3Squaring both sides, we geta+b2=3a2+b2+2 a. b=31+1+2 a. b=3 (Because a and b are unit vectors)2+ 2 a. b=32 a. b=12 a. b=1 a. b=12... 1Now,2a-5b.3a+b=6a2+2a. b-15 b. a-5b2=6a2+2a. b-15 a. b-5b2 (a. b=b. a)=6a2-13a. b-5b2=61-13 12-51 From (1)=1-132=-112

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