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Home  >>  CBSE XII  >>  Math  >>  Vector Algebra
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Find $|\overrightarrow x|$, if for a unit vector $\overrightarrow a,\:(\overrightarrow x-\overrightarrow a).(\overrightarrow x+\overrightarrow a)=15$

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  • $(\overrightarrow x-\overrightarrow a).(\overrightarrow x+\overrightarrow a)=|\overrightarrow x|^2-|\overrightarrow a|^2$
Given: $\overrightarrow a$ is a unit vector and
$(\overrightarrow x-\overrightarrow a).(\overrightarrow x+\overrightarrow a)=15$
$\Rightarrow\:|\overrightarrow x|^2-|\overrightarrow a|^2=15$
$\Rightarrow\:|\overrightarrow x|^2=15+1=16$
$\Rightarrow\:|\overrightarrow x|=4$
answered Feb 3, 2014 by rvidyagovindarajan_1
 

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