



 
Hi Dinesh, You are correct. If line l_{1} has slope m_{1} and line l_{2} has slope m_{2} the l_{1} and l_{2} are perpendicular if and only if m_{1} × m_{2} = 1. The difficulty in applying this to vertical lines like the yaxis is that a vertical line don't have a slopes. A horizontal line like the xaxis has a slope of zero but a vertical line doesn't have a slope. I hope this helps,  


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