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The value of c so that for all real x, the vectors $cx\vec i-6\vec j+3\vec k,\,x\vec i+2\vec j+2cx\vec k$ make an obtuse angle are: |
c < 0 0 < c < 4/3 -4/3 < c < 0 c > 0 |
-4/3 < c < 0 |
$cx^2-12+6cx<0$ $∀x∈R$ $D<0$ & $c<0$ $36c^2+48c<0$ $⇒-\frac{4}{3}<c<0$ |
