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Answer: -3x+y-4 = 0 (standard form)</h3>
This is equivalent to y = 3x+4 (slope intercept form)
By "standard form", I mean the form Ax+By+C = 0.
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Explanation:
Let's solve the first equation for y
x+3y-4 = 0
x+3y = 4
3y = 4-x
3y = -x+4
y = (-x+4)/3
y = (-x/3) + (4/3)
y = (-1/3)x + (4/3)
The equation is now in y = mx+b form, aka slope intercept form, with m = -1/3 as the slope and b = 4/3 as the y intercept. We'll focus on the slope.
Apply the negative reciprocal to this so that we go from -1/3 to +3/1 or simply 3. Flip the fraction and the sign. Note how -1/3 and 3 multiply to -1. Perpendicular slopes always multiply to -1, assuming neither line is vertical.
So this mystery perpendicular line we're after has a slope of 3.
It has the same y intercept as 2x+5y-20 = 0. Plug in x = 0 and solve for to determine the y intercept.
2x+5y-20 = 0
2(0)+5y-20 = 0
5y-20 = 0
5y = 20
y = 50/5
y = 4
The y intercept of 2x+5y-20 = 0 is y = 4, so it's also the y intercept of our final answer. Let b = 4.
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We found that:
- m = 3 is the slope of the perpendicular line
- b = 4 is the y intercept of the perpendicular line
So we know that,
y = mx+b
y = 3x+4
is the slope intercept form of the answer. Since your teacher gave you the equations in standard form (one version of it anyway), let's convert y = 3x+4 to that form as well
y = 3x+4
y-3x = 4
-3x+y = 4 .... one way to express standard form
-3x+y-4 = 0 .... another standard form
Some math textbooks use Ax+By = C as standard form, while others use Ax+By+C = 0. Unfortunately, it's a bit confusing because the same phrasing is used.
