Using "construct perpendicular to" tool.
1. Set snap mode to nearest2..Keyin 'construct perpendicular to'3. Select first element i.e. select a line4. Hit and hold the Ctrl+Shift keys5. Select second element i.e. parallel lineThe Ctrl+Shift allows the tool to use accusnap for element placement
https://communities.bentley.com/communities/other_communities/askinga/w/askinga/how-to-draw-a-line-perpendicular-to-an-arc-at-a-specific-point.aspx
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