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Is line y = kx + b tangent to circle x^2 + y^2 = 1 ?

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Is line y = kx + b tangent to circle x^2 + y^2 = 1 ? [#permalink] New post 06 Jun 2012, 10:33
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Is line y = kx + b tangent to circle x^2 + y^2 = 1 ?

(1) k + b = 1
(2) k^2 + b^2 = 1
[Reveal] Spoiler: OA
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Re: Line y = kx + b tangent to circle [#permalink] New post 06 Jun 2012, 10:35
manulath wrote:
Is line y=kx+b tangent to circle x^2+y^2=1 ?
(1) k+b=1
(2) k^2+b^2=1


I was unable to grasp the explanation. Kindly help me with detailed explanation
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Re: Line y = kx + b tangent to circle [#permalink] New post 06 Jun 2012, 23:47
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Hi,

To check the tangency of line to a circle:
if the shortest distance from the center of the circle to the line is equal to radius then the line is tangent to circle.

center of circlex^2+y^2=1 is (0,0)

For line y-kx-b=0 to be tangesnt, the shortest distance between line & (0,0) = radius = 1
or |(0-k*0-b)/\sqrt{1+k^2}|=1
or |b|=\sqrt{1+k^2}
squaring both sides,
b^2=1+k^2
so, question can be reframed as Is b^2-k^2=1?

Clearly, neither (1) nor (2) proves the above relation.

Hence, answer is (E)
(Note that for some value of b & k the line might be tangent, but using (1) or (2) we can't definitely say so)
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Last edited by cyberjadugar on 07 Jun 2012, 03:58, edited 1 time in total.
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Re: Line y = kx + b tangent to circle [#permalink] New post 07 Jun 2012, 00:41
cyberjadugar wrote:
For line y-kx-b=0 to be tangesnt, the shortest distance between line & (0,0) = radius = 1
or |(0-k*0-b)/\sqrt{1+k^2}|=1
or |b|=\sqrt{1+k^2}
squaring both sides,
b^2=1+k^2


It would be really helpful if you can explain the above.
How that particular equation was arrived at?
Thanks
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Re: Line y = kx + b tangent to circle [#permalink] New post 07 Jun 2012, 00:54
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Hi,

This is the formula from 10+ maths,

to find the shortest distance of a point (x_1,y_1) from line y=mx+c
change the equation of line to y-mx-c=0
then the distance d = |(y_1-mx_1-c)/\sqrt{(coff. of. y)^2+(coff. of. x)^2}|
thus, d = |(y_1-mx_1-c)/\sqrt{(1)^2+(-m)^2}|
I could not think of any other direct approach for now.

Regards,
manulath wrote:
cyberjadugar wrote:
For line y-kx-b=0 to be tangesnt, the shortest distance between line & (0,0) = radius = 1
or |(0-k*0-b)/\sqrt{1+k^2}|=1
or |b|=\sqrt{1+k^2}
squaring both sides,
b^2=1+k^2


It would be really helpful if you can explain the above.
How that particular equation was arrived at?
Thanks

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Re: Line y = kx + b tangent to circle [#permalink] New post 07 Jun 2012, 01:25
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cyberjadugar wrote:
Hi,

This is the formula from 10+ maths,

to find the shortest distance of a point (x_1,y_1) from line y=mx+c
change the equation of line to y-mx-c=0
then the distance d = |(y_1-mx_1-c)/\sqrt{(coff. of. y)^2+(coff. of. x)^2}|
thus, d = |(y_1-mx_1-c)/\sqrt{(1)^2+(-m)^2}|
I could not think of any other direct approach for now.



This is a sure shot winner. A helpful formula and time saver.
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Re: Line y = kx + b tangent to circle [#permalink] New post 07 Jun 2012, 06:32
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manulath wrote:
Is line y=kx+b tangent to circle x^2+y^2=1 ?
(1) k+b=1
(2) k^2+b^2=1

OA:
[Reveal] Spoiler:
E

Although the other solutions are mathematically correct, I think there's an easier way than memorizing a formula.

We need to solve for k and b.

1. A single equation does not allow us to solve for two variables. Additionally, k=1, b=0 gives an answer of no and k=0, b=1 gives an answer of yes. Insufficient.
2. See 1. Insufficient.

1+2. Taken together we have:
b=1-k
b^2=1-k^2
=> b^2=(1-k)^2=1-2k+k^2=1-k^2
==> 2k^2-2k=0 => k(2k-2)=0, and therefore k=0 or k=1.
In the case of k=0, b=1. Then the equation for the line is y=1, and the answer is YES. In the case of k=1, b=0. Then the equation for the line is y=x, and the answer is NO. Insufficient.

E.
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Re: Is line y=kx+b tangent to circle x^2+y^2=1 ? [#permalink] New post 07 Jun 2012, 16:45
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Is line y=kx+b tangent to circle x^2+y^2=1?

(1) k+b=1
(2) k^2+b^2=1

Notice that a circle represented by the equation x^2+y^2=1 is centered at the origin and has the radius of r=\sqrt{1}=1.

(1) k+b=1 --> if k=0 and b=1 then the equation of the line becomes y=1 and this line is tangent to the circle but if k=1 and b=0 then th equation of the line becomes y=x and this line is NOT tangent to the circle. Not sufficient.

(2) k^2+b^2=1. The same example is valid for this statement too. Not sufficient.

(1)+(2) Again the same example satisfies both statement: if k=0 and b=1 then the equation of the line becomes y=1 and this line is tangent to the circle but if k=1 and b=0 then th equation of the line becomes y=x and this line is NOT tangent to the circle. Not sufficient. Look at the diagram below to see both cases:
Attachment:
Tangent.png
Tangent.png [ 15.97 KiB | Viewed 2080 times ]


Answer: E.
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Re: Is line y=kx+b tangent to circle x^2+y^2=1 ?   [#permalink] 07 Jun 2012, 16:45
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