some easy vector problems #3!

Find the set of values of c for which the angle between the vectors cxi-6j+3k and xi-2j+2cxk is acute for every xÎ¾R

1 Answers

cxi-6j+3k and xi-2j+2cxk

for acute, you wnat angle <90

so you want cos theta as positive..

you want a.b >0

cx²+6cx+12>0

find the valu eof c such that this inequality holds for all x?

1) c>0

2then D<0

36c²<48c

so c<4/3

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