Posted by **John** on Monday, October 15, 2007 at 10:39pm.

Still need help on this one

Drawing a blank on this one, not sure where to start.

Find the equation, in standard form, with all integer coefficients, of the line perpendicular to x + 3y = 12 and passing through (9, -5).

- Algebra -
**Reiny**, Monday, October 15, 2007 at 10:51pm
Very easy

Recall that the slopes of perpendicular lines have slopes that are negative reciprocals of each other, or in other words, the product of their slopes is -1

Your first line is x + 3y = 12

So the perpendicular line must be

3x - y = c , the only thing we don't know is the constant

But (9,-5) lies on this new line

so 3(9) - (-5) = c

c = 32

The equation must be 3x - y = 32

- Algebra -
**Reiny**, Monday, October 15, 2007 at 11:43pm
(continued)

so the slope of the new line must be +3

then the new equation is y = 3x + b

but (9,-5) lies on it

so -5 = (3)(9) + b

b = -32

for y = 3x - 32

- Algebra -
**John**, Tuesday, October 16, 2007 at 9:04am
Thank you so much.

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