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# C1 Circle Questions Watch

1. ======================

1) I got the answer fairly easily , which is
x^2 + y^2 -4x - 2y - 95 =0

2) But for number 2, I sub in the x value of 5 and the y value of k to get :
25 + k^2 -20 - 2k - 95 =0 , which can be rearranged to k^2 - 2k - 90 = 0

I find it difficult though to use the quadratic formula. I get until 2k +/- root 2k^2 +360k over 2k and dont know what to do next.

3) How can you tell if it lies inside/ out ?
2. When you use the quadratic formula, you don't include the k's and k^2's - you just need to know how many there are. So the equation would be (2 +/- sqrt(2^2+4*90))/2, which comes out to 1 +/- sqrt(91).

As for the third part, I would use pythagoras to find out the distance from the centre to (-3,9). If it's less than the radius then it's inside the circle, if not it's outside.
3. (Original post by Arbolus)
When you use the quadratic formula, you don't include the k's and k^2's - you just need to know how many there are. So the equation would be (2 +/- sqrt(2^2+4*90))/2, which comes out to 1 +/- sqrt(91).

As for the third part, I would use pythagoras to find out the distance from the centre to (-3,9). If it's less than the radius then it's inside the circle, if not it's outside.

Oh yh, I didn't realise that the letter k takes the place of x.

Edit: I'm now stuck on this part . I end up with 2+/- root 4 + 360 over 2 . Cancel out the 2 from the bottom to give:
1+/- root182
4. (Original post by Gelato)
Oh yh, I didn't realise that the letter k takes the place of x.

Edit: I'm now stuck on this part . I end up with 2+/- root 4 + 360 over 2 . Cancel out the 2 from the bottom to give:
1+/- root182
To simplify a surd you need to take out the largest square factor you can. So:

root(364) = root(4*91) = root(4)*root(91) = 2*root(91).

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Updated: December 21, 2010
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