I know that to find the perpendicular of a linear line of form $y = m x + c$, you create a new equation with a gradient of the negative reciprocal $-1/m$. This is how you change it by 90 degrees. Is there a way of doing this by 30 degrees say? Is there a formula for rotating lines?
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Do you know how to rotate coordinates? – J. M. ain't a mathematician Mar 30 '17 at 23:03
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One way is to use the formula for the tangent of the sum of two angles. – amd Mar 31 '17 at 00:15
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The angle is given by $\arctan(m)$. – Mar 31 '17 at 03:50