交面式曲线方程 F = (x^2+y^2)/4 - z, G = y - 4
在点 (2, 4, 5), n1 = (Fx, Fy, Fz) = (x/2, y/2, -1) = (1, 2, -1),
n2 = (Gx, Gy, Gz) = (0, 1, 0)
切线向量 s = n1×n2 = (1, 0, 1),
对于 x 轴的倾角 t = arccos(1/√2) = π/4
5.z=x^2/4+4,
z'=x/2,
在点(2,4,5)处,z'=1,
∴所求切线对x轴的倾角是π/4.