z=[function(oppartial(y),[(8*y+18*x)/sqrt([8*y+18*x]^2+[9*y+19*x]^2)])]+[function(oppartial(x),[(-(9*y)-(19*x))/sqrt([8*y+18*x]^2+[9*y+19*x]^2)])]

function(optotal(x),y)=(8*y+18*x)/(-(9*y)-(19*x))

Graph of the formula

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