Let `vec(a)` , `vec(b)` and `vec(c)` be three non-zero vectors such that no two of them are collinear and `(vec(a)×vec(b))×vec(c)=1/3|vec(b)||vec(c)|vec(a)`. If `theta` is the angle between vectors `vec(b)` and `vec(c)`, then the value of `sintheta` is: