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Let $k$ be a positive real number. The line $x + y = 3 + k$ and the circle $x^2 + y^2 = k$ are drawn. Find $k$ so that the line is tangent to the circle.

 Jul 14, 2024
 #1
avatar+129850 
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Slope of line = -1

Slope of any tangent line to the circle=   -x/y

So

-x/y = -1

x/y =1

x = y

 

But 

x + y =3 + k

x^2 + y^2 = k

 

So

x + x  =  3+k

x^2 + x^2 = k

 

So

 

2x = 3+k    →  x = (3 + k) / 2

 

So

 

2x^2  = k

2 [ (3 + k) / 2] ^2  = k

(3 + k)^2  = 2k

k^2 + 6k + 9 = 2k

k^2 + 4k+ 9  = 0

The discriminant =  16 - 4 * 1 * 9  =   -20

So....k is  not a real value

So....no real solution

 

cool cool cool

 Jul 14, 2024

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