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