Explanation
Since m (gradient) and x1 are fixed and y1 is variate, then the formed a set of parallel lines because gradient of every lines remains 'm' .
Since the distance between the parallel line lx + my + n = 0 and lx + my + n’ = 0 is same as the distance between parallel lines mx + ly + n = 0 and mx + ly + n’ = 0. Therefore the parallelogram is a rhombus. Since the diagonals of a rhombus arc at right angles, therefore the required angle is π/2
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