Question

last day before exam need help immediately.

Answer 1

A.B and A.C tells us that A is perpendicular to both B and C. Now B cross C will give vector perpendicular to them. As they are unit vectors, magnitude of B cross C will be 1/2 and this vector will be parallel to A. As we don't know which side it is pointing towards, i.e. it might be in the direction of A or opposite to it. That's why plus or minus 2 times B cross C gives A. Hence proved.