32.
du/dx = 12x² + 9x²y + 3y²
du²/(dx*dy) = 9x² + 6y
33.
du/dx = y + cos(x+y)
du²/(dx*dy) = 1 - sin(x+y)
34.
du/dx = 1/(cos²(x+y)*tg(x+y)) = 1/(cos(x+y)sin(x+y)) = 2/sin(2(x+y))
du²/(dx*dy) = -4cos(2(x+y))/sin²(2(x+y))
35.
du/dx = 1/(1+(x+y)²/(1-xy)²) * ((1-xy) + y(x+y))/(1-xy)² = (1+y²)/(1+x²+y² + x²y²)
du²/(dx*dy) = (2y(1+x²+y²+x²y²) - 2y(1+x²)(1+y²))/(1+x²+y²+x²y²)² = 0