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In the xy-plane, line l passes through the origin and is perpendicular to the line 3x +...

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ogidi | Student | (Level 1) Honors

Posted November 25, 2012 at 7:59 PM via web

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In the xy-plane, line l passes through the origin and is perpendicular to the line 3x + 2y = k. k is constant.If the 2 lines intersect at (t, t + 1), what is t?

Im confused for the fact that I dont know the value of k. 

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mlehuzzah | Student, Graduate | (Level 1) Associate Educator

Posted November 25, 2012 at 10:59 PM (Answer #1)

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You don't actually need k:

3x+2y=k

You can rewrite this as:

2y=-3x+k , or:

y = -1.5 x + (k/2)

 

The slope of this line is -1.5 (or -3/2)

Line l is perpendicular to this line, which means it has slope +2/3 (the negative reciprocal of -3/2)

Line l passes through the origin, so it has y-intercept of 0

So, line l has equation:

y = (2/3)x + 0

or just

y = (2/3)x

 

If the two lines intersect at (t,t+1), then l has to go through the point (t,t+1).  That means, we can plug in x=t and y=t+1, and the equation should be true.  So:

(t+1)=(2/3)(t)

multiplying both sides by 3:

3t+3 = 2t

1t+3=0

t=-3

 

Thus, t=-3.

 

PS

You could, from here, if you wanted, use all of this information to figure out k, since you now know that the first line must pass through the point (-3,-2).  But you don't actually need k to figure out t.

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