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Homework help! (straight line)

The straight lines y=6x+1 and x-ky+4=0 are perpendicular to each other. Find the value of k.
I have got the perpendicular gradient which is -1/6.
What do I do next ?
Original post by Mystery.
The straight lines y=6x+1 and x-ky+4=0 are perpendicular to each other. Find the value of k.
I have got the perpendicular gradient which is -1/6.
What do I do next ?


Rewrite the equation of the second line in the form y = .......
Whatever is in front of x must equal -1/6
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Mystery.
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Original post by tiny hobbit
Rewrite the equation of the second line in the form y = .......
Whatever is in front of x must equal -1/6


I did that but when I rearrange it back, I get x+6y—4 =0 whereas in the in total question it is ADD 4 not SUBTRACT 4?
Original post by Mystery.
I did that but when I rearrange it back, I get x+6y—4 =0 whereas in the in total question it is ADD 4 not SUBTRACT 4?


You've made an error somewhere. k = - 6. When you rearrange it to get rid of the fractions you should come up with the right signs.
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Original post by tiny hobbit
You've made an error somewhere. k = - 6. When you rearrange it to get rid of the fractions you should come up with the right signs.


Okay if I do it that way, the answer works out to be x+6y+4=0, however in the question its is —ky, does that mean k=-6? Am I doing something wrong?
Original post by Mystery.
Okay if I do it that way, the answer works out to be x+6y+4=0, however in the question its is —ky, does that mean k=-6? Am I doing something wrong?


I'm slightly lost...

Rearranging the second equation gives y = x/k + 4/k line 1

Comparing with your -1/6 gives 1/k = -1/6 so k = -6 line 2

Put that back in line 1 gives y = - 1x/6 -4/6

Multiplying by 6 gives 6y = -x -4

Take it to one side and you get x + 6y + 4 = 0

The last part is just a check, the answer is produced in line 2.
Reply 6
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Mystery.
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Original post by tiny hobbit
I'm slightly lost...

Rearranging the second equation gives y = x/k + 4/k line 1

Comparing with your -1/6 gives 1/k = -1/6 so k = -6 line 2

Put that back in line 1 gives y = - 1x/6 -4/6

Multiplying by 6 gives 6y = -x -4

Take it to one side and you get x + 6y + 4 = 0

The last part is just a check, the answer is produced in line 2.


Thank you but my question was :

In the initial question the form is x-ky+4=0 (MINUS ky)
However, in the answer it works out to be x+ky+4=0 (PLUS ky)

Does that mean k=6 or k=-6?
Original post by Mystery.
Thank you but my question was :

In the initial question the form is x-ky+4=0 (MINUS ky)
However, in the answer it works out to be x+ky+4=0 (PLUS ky)

Does that mean k=6 or k=-6?


k = - 6, because then - k = - - 6 which is + 6.
Reply 8
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Original post by tiny hobbit
k = - 6, because then - k = - - 6 which is + 6.

Thanks!

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