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| Parallel and perpendicular lines |
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| Let l1 and l2 be any two lines intersecting at P. |
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| Let line l1 be inclined at an angle q1 and line l2 be inclined at an angle q2 with the x-axis in the positive direction. |
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| Let f be the angle between the lines. |
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| m1 = slope of line l1, m2 = slope of line l2 |
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| From the figure, |
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| l1 is parallel to l2 if f = 900. |
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| \Two lines are parallel if their slopes are equal. |
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Two lines are perpendicular if the product of their slopes is -1 or slope
of one line is the negative reciprocal of the other i.e.,  |
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| Example: |
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Show that the line through (-6,6) and (-1,-3) is perpendicular
to the line through (2,-4) and  |
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| Suggested answer: |
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| Since, the product of the slopes = -1, the lines are perpendicular to each other. |
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