An analytic function of a complex variable z = x + iy is expressed as f(z) = u, (x,y) + iv(x,y) where i = `sqrt(-1)` . If u = xy, the expression of v should be
`((x+y)^2)/(2)+k`
`(x^2 -y^2)/(2)+k`
`(y^2 -x^2)/(2)+k`
`((x-y)^2)/(2)+k`
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