If b is the direction of the line and n is the normal to the plane, the angle phi between the line and the plane (complement of angle with normal) is: sin(phi) = |b.n|/(|b||n|). The line is parallel to the plane when b.n = 0 (since sin(phi) = 0 means phi = 0). The line is perpendicular to the plane when b is parallel to n (i.e., b x n = 0). Common JEE trap: using cos instead of sin for this formula.