MHB How Can I Find the Equation of the Dotted Tangent Line of a Circle?

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To find the equation of the dotted tangent line of the circle, first confirm the correct coordinates of point A, which should be (6,3) instead of (3,6). The tangent line is parallel to the lower tangent, given as y=0.5x, meaning it will also have the same slope. The relationship between the coordinates of point B (the tangent point) and point A can be established with the equation of the line segment BA, which is perpendicular to the tangent line. By substituting the derived expressions into the circle's equation, a quadratic equation can be formed to solve for the tangent point. The discussion emphasizes the importance of accurate coordinates and the use of analytical geometry principles to derive the tangent line.
Yankel
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Dear all,

Attached is a picture of a circle.

View attachment 9133

The lower tangent line is y=0.5x. The center of the circle is M(4,7) while the point A is (3,6).

I found the equation of the circle, it is:

$(x-4)^{2}+(y-7)^{2}=20$

and I wish to find the dotted tangent line. I know that it is parallel to the lower one, therefore, the slope is the same. I can't find the tangent point.

Can you give me a hint please ?
 

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Yankel said:
Dear all,

Attached is a picture of a circle.
The lower tangent line is y=0.5x. The center of the circle is M(4,7) while the point A is (3,6).

I found the equation of the circle, it is:

$(x-4)^{2}+(y-7)^{2}=20$

and I wish to find the dotted tangent line. I know that it is parallel to the lower one, therefore, the slope is the same. I can't find the tangent point.

Can you give me a hint please ?

You know $A=(3,6)$, and a direction vector along the lower tangent line is $\langle 2,1\rangle$. Can you find a vector perpendicular to that vector and pointed upwards towards the other tangent? And make it $\sqrt{20}$ units long? Then just add its components to $(3,6)$ to get the other tangent point.

[Edit] I notice that your point A doesn't satisfy the equation of your circle, so check your work so far.
 
Last edited:
I am sorry, I did not mention this. The problem is in analytical geometry, no vectors are allowed
 
Yankel said:
I am sorry, I did not mention this. The problem is in analytical geometry, no vectors are allowed

Since the tangent line is parallel, it must be of the form $y=\frac 12 x + b$ for some constant $b$, which is also the intersection with the y-axis.

Substitute in the circle equation, which becomes a quadratic equation.
Since it is a tangent it has exactly 1 point on the circle.
This happens when the square root part of the quadratic solution is zero.
From there we can find $b$.
 
Something is wrong here. If a circle has centre at $(4,7)$ and goes through the point $A$ at $(3,6)$, then the tangent at $A$ will not go through the origin. It looks to me as though $A$ should be the point $(6,3)$ rather than $(3,6)$.
 
Hi Yankel.

As Opalg pointed out, there is a typo with the co-ordinates of the point A; it should be $(6,3)$ rather than $(3,6)$.

Let B with co-ordinates $(u,v)$ be the point opposite A on the circle. Then the line segment BA is perpendicular to the tangent line $y=\frac12x$ and so has gradient $-2$, i.e.
$$\frac{v-3}{u-6}\ =\ -2$$
from which we get
$$v\ =\ -2u+15\quad\ldots\boxed1.$$
Also $(u,v)$ lies on the circle, so
$$(u-4)^2+(v-7)^2\ =\ 20\quad\ldots\boxed2.$$
Substitute $v$ from $\boxed1$ into $\boxed2$ will give you a quadratic equation in $u$, which you can easily solve (knowing that $u=6$ is one solution).
 
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