for a partical moving in a circle of radius r, centripetal acceleration is a(vector) = w(vector) x v(vector) why is it so? why cant we write a(vector) = v(vector) x w(vector) ?

First thing, A.B=B.A,

but AXB is not equal to BXA.  Rather, AXB=-(BXA).

So, in ur question, the direction of the acceleration will change from -ve axis to +ve axis.

sorry but, your ans. was not clear.

I asked about rotation motion.

the expresion for acceleration vector is wrong by you. centripetal acc. isa= M*R*W2 even the tangential acc. is a=r@ @ is angular acc.