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Equation of a line tangent to a circle (slope of the line is given)

ÀÛ¼ºÀÚ Uploader : mangsteen ÀÛ¼ºÀÏ Upload Date: 2018-09-17º¯°æÀÏ Update Date: 2018-09-17Á¶È¸¼ö View : 317

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When the slope of a line is given, the equation of a line tangent to
a circle is obtained as below.

Equation of the circle : (x+x0)^2 + (y+y0)^2 = R
Equation of the line : y = ax + k

Given : x0, y0, R, a
Unknown : k

From the equatuin of the line,

-ax+y-k = 0

Distance bewteen the line and the center of circle :

R^(1/2) = |(-a)*(-x0)+(-y0)-k|/((-a)^2+1)^(1/2)

|(-a)*(-x0)+(-y0)-k| = (R*((-a)^2+1))^(1/2)


¡á If (-a)*(-x0)+(-y0)-k > 0,

    (-a)*(-x0)+(-y0)-k = (R*((-a)^2+1))^(1/2)




¡á If (-a)*(-x0)+(-y0)-k < 0,

    (-a)*(-x0)+(-y0)-k = -(R*((-a)^2+1))^(1/2)


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