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### Question Asked by a Student from EXXAMM.com Team

** Q 2250667514. ** Let the vectors `vec(a) , vec(b) , vec(c)` and `vec(d)` be such that

`( vec(a) xx vec(b) ) xx ( vec(c) xx vec(d) ) = vec(0)`. Let `p_1` and `p_2` be planes

determined by the pairs of vectors `vec(a), vec(b)` and `vec(c) ,vec(d)`

respectively. Then the angle between `p_1` and `p_2`