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What is the radius of x^2 + y^2 -2x -4y -31 = 0

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First we have to complete the square on both the x and y terms. Move the 31 to the right side.
x^2 - 2x + y^2 - 4y = 31
x^2 - 2x + 1 + y^2 - 4y + 4 = 31 + 1 + 4
(x - 1)^2 + (y - 2)^2 = 36
36 = r^2
r = 6
User Alex Gidan
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