f(x, y) = x·y^3/(x^2 + y^2)
f'(x, y) = [y^3·(y^2 - x^2)/(x^2 + y^2)^2, x·y^2·(3·x^2 + y^2)/(x^2 + y^2)^2]
f''(x,y) = [2·x·y^3·(x^2 - 3·y^2)/(x^2 + y^2)^3, - y^2·(3·x^4 - 6·x^2·y^2 - y^4)/(x^2 + y^2)^3; - y^2·(3·x^4 - 6·x^2·y^2 - y^4)/(x^2 + y^2)^3, 2·x^3·y·(3·x^2 - y^2)/(x^2 + y^2)^3]
t(x, y) = f'(1, 1)·[x, y] + f(1, 1) = y + 1/2