修改梯度下降函数

def gradient_descent3(f,xstart,ystart,zstart,tolerance=1e-6,max_steps=1000):
    x = xstart
    y = ystart
    z = zstart
    grad = approx_gradient3(f,x,y,z)
    steps = 0
    while length(grad) > tolerance and steps < max_steps:
        x -= 0.01 * grad[0]
        y -= 0.01 * grad[1]
        z -= 0.01 * grad[2]
        grad = approx_gradient3(f,x,y,z)
        steps += 1
    print(steps)
    return x,y,z