Производная функции 10 баллов!!! Выбор лучшего ответа обеспечен!

Решение:
dz/dx = {[√(x² + y²) - 2x² / 2√(x² + y²)] / (x² + y²)} * {1 / √[1 - x² / (x² + y²)]} = (x² + y² - x²) / (x² + y²)^(3/2) * {1 / √[1 - x² / (x² + y²)]} = y² / {(x² + y²)^(3/2) * √[1 - x² / (x² + y²)]} = y² / {(x² + y²)^(3/2) * [(x² + y² - x²) / (x² + y²)]^(1/2)} = y² / {(x² + y²)^(3/2) * y / (x² + y²)^(1/2)} = y / {(x² + y²)^(3/2) / (x² + y²)^(1/2)} = y / (x² + y²)
dz/dy = [2y * (-1/2) / (x² + y²)^(3/2)] * {1 / √[1 - x² / (x² + y²)]} = [-y / (x² + y²)^(3/2)] * {1 / [y / (x² + y²)^(1/2)]} = [-y / (x² + y²)^(3/2)] * {(x² + y²)^(1/2) / y} = -1 / (x² + y²)
d²z/dxdy = d (dz/dx) / dy = [(x² + y²) - y * 2y] / (x² + y²)² = (x² - y²) / (x² + y²)²
d²z/dydx = d (dz/dy) / dx = - (-2x) / (x² + y²) = 2x / (x² + y²)