可调立体：平行于y轴的抛物面截面

GeoGebra 命令及参数

1. Intersect(lower, h, (0, -8)) → C
2. Intersect(upper, h, (0, 9)) → B
3. Segment(C, B) → q
4. Intersect(upper, q, (0, 9)) → G
5. Midpoint(C, G) → T
6. Line(T, T + (1, 0, 0)) → l_1
7. Segment(H, I) → s_1
8. Midpoint(s_1) → E
9. Line(E, E + (1, 0, 0)) → r_1
10. Segment(J, K) → q_1
11. Midpoint(q_1) → F
12. Line(F, F + (1, 0, 0)) → c
13. Line(B, D) → i
14. Sphere(B, d) → e
15. Segment(C, B) → p
16. Textfield(upper) → InputBox1
17. Textfield(lower) → InputBox2
18. Textfield(a) → InputBox3
19. Textfield(b) → InputBox4
20. Segment(C, G) → k_1
21. Line(C_1 + (0, 0, k), C_1 + (0, 1, k)) → g_2
22. Parabola(T, g_2) → c_2
23. CurveCartesian(x(C), y(C_1) + (t * (y(B) - y(C))), k - ((t * (y(B) - y(C))))^(2) / ((4 * k)), t, -1 / 2, 1 / 2) → o
24. Surface(x(C), y(C_1) + ((v * t) * (y(B) - y(C))), k - (v * ((t * (y(B) - y(C))))^(2) / (((4 * k) * v))), v, 0, 1, t, -1 / 2, 1 / 2) → u
25. CurveCartesian(x(E), y(E) + (t * (y(H) - y(I))), f - ((t * (y(H) - y(I))))^(2) / ((4 * f)), t, -1 / 2, 1 / 2) → g
26. CurveCartesian(x(K), y(F) + (t * (y(J) - y(K))), l - ((t * (y(J) - y(K))))^(2) / ((4 * l)), t, -1 / 2, 1 / 2) → m
27. Surface(b, y(F) + ((v * t) * (y(J) - y(K))), l - (v * ((t * (y(J) - y(K))))^(2) / (((4 * l) * v))), v, 0, 1, t, -1 / 2, 1 / 2) → n
28. Surface(a, y(E) + ((v * t) * (y(H) - y(I))), f - (v * ((t * (y(H) - y(I))))^(2) / (((4 * f) * v))), v, 0, 1, t, -1 / 2, 1 / 2) → r
29. Surface(u, (0.5 * (upper(u) + lower(u))) + (v * (upper(u) - lower(u))), (0.25 * (upper(u) - lower(u))) - ((v * (upper(u) - lower(u))))^(2) / (upper(u) - lower(u)), u, a, b, v, -1 / 2, 1 / 2) → t
30. CurveCartesian(t, (0.5 * (upper(t) + lower(t))), (0.25 * (upper(t) - lower(t))), t, a, b) → s