Mathematics

Problem

If the three planes

r \cdot n_1 = p_1, r \cdot n_2 = p_2

and

r \cdot n_3 = p_3

have a common line of intersection, then

p_1(n_2 \times n_3) + p_2(n_3 \times n_1) + p_3(n_1 \times n_2)

is equal to a. 1 b. 2 c. 0 d.

-1