Question

Find an equation of the line. Write the equation using function notation.

Through (5,1); perpendicular to 9y = x - 18

Answer 1

keeping in mind that perpendicular lines have negative reciprocal slopes, let's check for the slope of the equation above

`9y=x-18\implies y=\cfrac{x-18}{9}\implies y=\stackrel{\stackrel{m}{\downarrow }}{\cfrac{1}{9}}x-2\impliedby \begin{array}c \cline{1-1} slope-intercept~form\\ \cline{1-1} \\ y=\underset{y-intercept}{\stackrel{slope\qquad }{\stackrel{\downarrow }{m}x+\underset{\uparrow }{b}}} \\\\ \cline{1-1} \end{array} \\\\[-0.35em] ~\dotfill`

`\stackrel{~\hspace{5em}\textit{perpendicular lines have \underline{negative reciprocal} slopes}~\hspace{5em}} {\stackrel{slope}{ \cfrac{1}{9}} ~\hfill \stackrel{reciprocal}{\cfrac{9}{1}} ~\hfill \stackrel{negative~reciprocal}{-\cfrac{9}{1} \implies -9}}`

so we're really looking for the equation of a line whose slope is -9 and it passes through (5 , 1)

`(\stackrel{x_1}{5}~,~\stackrel{y_1}{1})\hspace{10em} \stackrel{slope}{m} ~=~ - 9 \\\\\\ \begin{array} \cline{1-1} \textit{point-slope form}\\ \cline{1-1} \\ y-y_1=m(x-x_1) \\\\ \cline{1-1} \end{array}\implies y-\stackrel{y_1}{1}=\stackrel{m}{- 9}(x-\stackrel{x_1}{5}) \\\\\\ y-1=-9x+45\implies {\Large \begin{array}{llll} y=-9x+46 \end{array}}`