The angle between vectors(adsbygoogle = window.adsbygoogle || []).push({}); aandbis cos^-1(4/21). Find p ifa= [6,3,-2] andb= [-2, p, -4]

I did:

cos x= 4/21 =a.b/|a||b|

The result comes out to be p=8/3 but it only satisfies that a dot b is 4 and not |a||b| = 21...What am I doing wrong?

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# Dot product

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