# Math Help - circle geomtery help

1. ## circle geomtery help

The circle C has centre (5, 2) and passes through the point (7, 3).

(a) Find the length of the diameter of C.
$2x \sqrt{4+1} = 2\sqrt{5}$

(b) Find an equation for C.
$(x-5)^2 + (y-2)^2 = 5$

(c) Show that the line y = 2x − 3 is a tangent to C and find the coordinates
of the point of contact.

Need help with question 'c'.

I know that the angle between a tangent and radius is 90 , but how would I show that the product of their gradients is -1, if I dont know the point of contact?

Also how do I find the point were it touches the circle?

2. Originally Posted by Tweety
The circle C has centre (5, 2) and passes through the point (7, 3).

(a) Find the length of the diameter of C.
$2x \sqrt{4+1} = 2\sqrt{5}$

(b) Find an equation for C.
$(x-5)^2 + (y-2)^2 = 5$

(c) Show that the line y = 2x − 3 is a tangent to C and find the coordinates
of the point of contact.

Need help with question 'c'.

I know that the angle between a tangent and radius is 90 , but how would I show that the product of their gradients is -1, if I dont know the point of contact?

Also how do I find the point were it touches the circle?
Here's one approach out of many possible approaches: Show that the two equations

$y = 2x - 3$

$(x-5)^2 + (y-2)^2 = 5$

have only one solution.

3. Originally Posted by mr fantastic
Here's one approach out of many possible approaches: Show that the two equations

$y = 2x - 3$

$(x-5)^2 + (y-2)^2 = 5$

have only one solution.
So if I solved this simutaneously, wouldn't that mean that these to equations intersect at that one point, instead of touching?

4. Originally Posted by Tweety
So if I solved this simutaneously, wouldn't that mean that these to equations intersect at that one point, instead of touching?
If a line intersects a circle at one point then it is tangent to the circle at that point.