A few days ago
E

Circles and tangent lines?

There are two tangent lines from the point (0,1) to the circle x^2 + (y+1)^2 = 1. Find equations of these two lines by using the fact that each tangent line intersects the circle in exactly one point.

So far, i’ve drawn the circle and the two tangent lines that make a little hat on top of the circle. I want to use the slope formula from the point (0,1) to each side of the circle where the tangent lines meet it, but i dont know how to find those two points. A little help would be appreciated.

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A few days ago
stvenryn

Favorite Answer

You did good. So, you know that the circle is centered at (0,-1). Draw a circle at that point. Draw atangent from the point (0,1). Then draw line perpendicular to the tangent from the center of the circle which is located at (0,-1). Now you have a right angle triangle with a hypotinuse of “2” and a side of “1”. that means the angle subtended by the tangent to the y axis at (0,1) is about 30 degrees(you have to find out what the angle is when sin(a) =0.5). and the length of the tangent

l= sqrt (2^2 -1) =sqrt(3) =1.732.

Now draw a line perpendicular to the y axis to the point where the tangent cuts the circle. OK? The length of that line would be 1.732 sin(a), where “a” is the angle corresponding to sin(a) =0.5.

So your tangent cuts circle at [(1.732 sin(a) , 1-1.732 cos(a)]

and the second tangent will cut at [(-1.732sin(a),1-1.732cos(a)]

So your points are (0.866, -0.5) and (-0.866, -0.5)

You can also do this as a algebra problem. assume the equation of the tangent is y = cx +1 [ this passes through (0,1)]. Substitute this in your circle equation.

You get (c^2 +1)x^2 +c^2(x^2) +3 = 0 . Then you find the 2 roots of this qudratic equation as

x1 , x2 = {-4c +/- sqrt [16c^2 – 12(c^2 +1)]}/ (c^2+1)

Substitute in y = cx +1 and you get y1 and y2

So your points of intersection will be (x1,y1) and( x2,y2)

Substitute these points in the circle equation y = cx + 1 and you can evaluate c in terms of x1 and y1. It may get little tedious, but you can do it by this methos also.

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