SOLUTION: Find the equation of all lines passing through the point (6, -1) for which the product of their x- and y-intercepts is 3

Algebra ->  Linear-equations -> SOLUTION: Find the equation of all lines passing through the point (6, -1) for which the product of their x- and y-intercepts is 3       Log On


   



Question 1155733: Find the equation of all lines passing through the point (6, -1) for which the
product of their x- and y-intercepts is 3

Found 2 solutions by MathLover1, greenestamps:
Answer by MathLover1(20849) About Me  (Show Source):
You can put this solution on YOUR website!

given:
lines passing through the point (6, -1)
the
product of their x- and y-intercepts is 3

if the product of their x- and y-intercepts is 3, one intercept must be 1 and other must be 3
let’s find out which one is 1 and which one is 3

x%2Ay=3
x=3%2Fy
x-intercept at:(3%2Fy,0)
y-intercept at:(0,y)
then a slope of the line that passes through intercepts is:
m=%28y%5B2%5D-y%5B1%5D%29%2F%28y%5B2%5D-y%5B1%5D%29
m=%28y-0%29%2F%280-3%2Fy%29
m=y%2F%28-3%2Fy%29
m=y%5E2%2F-3
plug it in equation of the line:
y=mx%2Bb
y=%28y%5E2%2F-3%29x%2Bb......use given point to calculate b
-1=%28%28-1%29%5E2%2F-3%29%2A6%2Bb

-1=%281%2F-1%29%2A2%2Bb
-1=-2%2Bb
2-1=b
b=1-> if b=1 then y-intercept is 1
=> x-intercept must be 3
and m=y%5E2%2F-3=>m=1%5E2%2F-3=>m=-1%2F3
your equation will be:

y=-%281%2F3%29x%2B1



Answer by greenestamps(13200) About Me  (Show Source):
You can put this solution on YOUR website!


The solution by the other tutor is incomplete; there are two lines that satisfy the given conditions.

In her solution, she assumes that, since the product of the two intercepts is 3, one of the must be 3 and the other must be 1. That is not true.

Let the intercepts be (a,0) and (0,b).

The equation of the line is y = mx+b. We know
(1) m+=+-b%2Fa
(2) (6,-1) is on the line, so -1+=+%28-b%2Fa%29%286%29%2Bb

(3) ab=3

Simplify (2) and substitute (3):

-1+=+%28-b%2Fa%29%286%29%2Bb
-a+=+-6b%2Bab
-a+=+-6b%2B3
a+=+6b-3 (4)

Substitute (4) in (3):

%286b-3%29%28b%29+=+3
6b%5E2-3b-3+=+0
2b%5E2-b-1+=+0
%282b%2B1%29%28b-1%29+=+0
b+=+-1%2F2 or b+=+1

ANSWERS:

b=-1/2 and ab=3 means a = -6; the intercepts are (-6,0) and (0,-1/2); the equation is y = (-1/12)x-1/2

b = 1 and ab=3 means a=3; the intercepts are (3,0) and (0,1); the equation is y = (-1/3)x+1

A graph....

graph%28400%2C400%2C-10%2C10%2C-4%2C4%2C%28-1%2F12%29x-1%2F2%2C%28-1%2F3%29x%2B1%29