1.

If ` vec a`and ` vec b`aretwo non-collinear unit vectors such that `| vec a+ vec b|=sqrt(3),`find `(2 vec a-5 vec b)dot(3 vec a+ vec b)`.

Answer» Correct Answer - `(-11)/2`
Given : `|vec(a)|=1, |vec(b)|=1` ad `|vec(a)+vec(b)|=sqrt(3)`.
`:. |vec(a)+vec(b)|^(2)=(vec(a)+vec(b)).(vec(a)+vec(b))`
`=(vec(a).vec(a))+2(vec(a).vec(b))+(vec(b).vec(b))`
`= |vec(a)|^(2)+ 2(vec(a).vec(b))+|vec(b)|^(2)=2+2(vec(a).vec(b))`.
`:. 2[1+(vec(a).vec(b))]=3 implies (vec(a).vec(b))=1/2`.
Now, find `(2 vec(a)- 5 vec(b)).(3 vec(a)+vec(b))`.


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