The answers depend on the context. If QX is a vector, then two other ways of naming it are:
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All you do is look at what's closer to plane v so it's XQ or QX
The coordinates of the mid points are the means of the pairs of coordinates of the two end points. Thus, if P = (px, py, pz) and Q = (qx, qy, qz) then the midpoint of PQ is [(px+qx)/2, (py+qy)/2, (pz+qz)/2]. The result can be simplified for 2-dimensional space, or extended to more dimensions.