1.
The vectors a\vec{a} and b\vec{b} are not perpendicular and c\vec{c} and d\vec{d} are two vectors satisfying b×c=b×d\vec{b} \times \vec{c} = \vec{b} \times \vec{d} and ad=0\vec{a} \cdot \vec{d} = 0. Then the vector d\vec{d} is equal to