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# Coordinate geometry watch

1. A circle has a centre (2,-3) and the line y=2x+3 is a tangent. How do I work out the radius from this?
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2. (Original post by Millyflower)
A circle has a centre (2,-3) and the line y=2x+3 is a tangent. How do I work out the radius from this?
Thanks

Posted from TSR Mobile
Have you gotten anywhere with it at all? Focus on the 'tangent' part to start off with. (Draw a diagram if you think it would help)
3. i would probably try to find the equation of the radius at the point of tangency... which means you need to know the gradient of the radius.... but this is linked to the gradient of the tangent line.
4. Alternatively you can consider the discriminant of the equation you get when you try to find the points of intersection, to get the radius.

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