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# How to find the radius of a circle

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1. The centre of a circle I have is 6,8 Can i find out the radius from ths info?
2. (Original post by SunDun111)
The centre of a circle I have is 6,8 Can i find out the radius from ths info?
Only if you know another point lying on the circumference of the circle.
3. Not just from the centre, so I'm guessing you probably have an equation looking like (x-6)^2+(y-8)^2?
Those equations are in the form (x-a)^2+(y-b)^2=radius^2
Hope that helps
4. (Original post by Zacken)
Only if you know another point lying on the circumference of the circle.
I have the coordinates of the tangent, its 9,12?
5. (Original post by SunDun111)
I have the coordinates of the tangent, its 9,12?

How do you find the length of the blue line (the radius), you know how to use the distance between two points formula, don't you?
6. (Original post by SunDun111)
I have the coordinates of the tangent, its 9,12?
It's the hypotenuse of the triangle from the centre of the circle to the point. r=sqrt((x-a)^2+(y-b)^2)
7. (Original post by Zacken)

How do you find the length of the blue line (the radius), you know how to use the distance between two points formula, don't you?
(Original post by XOR_)
It's the hypotenuse of the triangle from the centre of the circle to the point. r=sqrt(x^2+y^2)
Did it thanks
8. (Original post by SunDun111)
Did it thanks
Cheers.
9. (Original post by SunDun111)
Did it thanks
sorry, error: r=sqrt((x-a)^2+(y-b)^2)
10. (Original post by Zacken)
Cheers.
a question on trig identities,

If i had CosxSinx (multiplied together) all over Cos X, do the Cos's cancel so its just Sinx.
11. (Original post by SunDun111)
a question on trig identities,

If i had CosxSinx (multiplied together) all over Cos X, do the Cos's cancel so its just Sinx.
Yes.

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