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x²+y²+2x+4y+1=0 cuts x²+y²-6x-4y+k=0 orthogonally. Then k equals
A-15
B15
C-7
D-1
Answer & Solution
Correct answer: A. -15
The condition 2g1g2 + 2f1f2 = c1 + c2 gives -6 - 8 = 1 + k, so k = -15.
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