1.

Show that the equaion ` x ^(2) + y ^(2) - 6x + 4y - 36 = 0 ` represents a circle. Also, find its centre and radius.

Answer» The given equation is ` x ^(2) + y ^(2) - 6x + 4y - 36 = 0` .
This is of the form ` x ^(2) + y^(2) + 2gx + 2 fy + c= 0`,
`" "` where ` 2g = - 6, 2f = 4 and c = - 36`
` therefore g = -3, f = 2 and c = - 36`.
Hence, the given equation represents a circle.
Centre of the circle `= (-g, -f ) = (3, - 2 )`
Radius of the circle = ` sqrt ( g ^(2) + f ^(2) - c ) = sqrt (3 ^(2) + (-2 ) ^(2) + 36 ) = sqrt ( 49) = 7 ` units.


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