Given `|veca|=|vecb|=1` and `|veca+vecb|=sqrt(3)`. If `vecc` be a vector such that `vecc-veca-2vecb=3(vecaxxvecb)`, then `vecc.vecb` is equal to

Explanation

We, have, `|veca + vecb|=sqrt3`
` or |veca + vecb|^(2) =3`
`or |veca|^(2) + |vecb|^(2) + 2(veca .vecb) =3`
`or veca. Vecb = 1//2` (i)
now ` vecc - veca - 2vecb = 3(veca xx vecb) `
`or (vecc - veca - 2vecb) .vecb = 3{(veca xx vecb) .vecb}`
`or vecc . vecb - 1/2 - 2xx 1 =0`
or `vecc.vecb = 5//2`