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If vec(a)andvec(b) are two unit vectors ...

If `vec(a)andvec(b)` are two unit vectors such that `vec(a) + 2vec(b) and 5vec(a) - 4vec(b)` are perpendicupar to each other, then the angle between `vec(a) and vec(b)` is

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`(vec(a)+2vec(b)).(5vec(a)-4vec(b))=0`
`5a^(2)4abcostheta+10abcostheta-8b^(2)=0`
`=5-4 cos theta+10costheta-8=0[a=b=1]`
`= - 3 + 6 cos theta = 0`
`cos theta = 1//2, theta = 60^(@)`
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