z := x^2-y^2; x^2+y^2=9; dS := sqrt(1+diff(z,x)^2+diff(z,y)^2); Answer := Int(Int(dS,y=-sqrt(9-x^2)..sqrt(9-x^2)),x=-3..3) = evalf( int(int(dS,y=-sqrt(9-x^2)..sqrt(9-x^2)),x=-3..3));