CircleHard

Question

If the circle x2 + y2 = 9 touches the circle x2 + y2 + 6y + c = 0, then c is equal to -

Options

A.-27
B.36
C.-36
D.27

Solution

     
S1 : x2 + y2 = 9   ⇒ C1(0, 0), r1 = 3
S2 : x2 + y2 + 6y + c = 0
⇒ C2 (0, -3), r2 = 
Now, C1C2 = r2 - r1
3 = - 3
36 = 9 - c    ⇒    c = - 27

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