find the angle between A=i-2j-5k and B=2i+j-4k

a=i-2j-5k b=2i+j-4k

Now, a.b=2-2+20=20

We know that, a.b=abcosθ cosθ=(a.b)÷ab

{a=√a.a, b=√b.b} [ Here, a=√30 and b=√21}

So, cosθ=20÷(√30.√21)

Thus, θ=cos^-1 [0.8] or θ= 37

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