Let a, b and c be distinct positive numbers. If the vectors $a\widehat i + a\widehat j + c\widehat k,\widehat i+\widehat k$ and $c\widehat i + c\widehat j + b\widehat k$ are co-planar, then c is equal to :
${2 \over {{1 \over a} + {1 \over b}}}$
${{a + b} \over 2}$
${1 \over a} + {1 \over b}$
$\sqrt {ab} $
Solution
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