三次函數的圖形
Author: 楊佳玲

d = 3.2 (slider from -5 to 5, step 0.1)
c = -3.2 (slider from -5 to 5, step 0.1)
b = 0.6 (slider from -5 to 5, step 0.1)
a = 1.4 (slider from -5 to 5, step 0.1)
f(x) = a * x^(3) + b * x^(2) + c * x + d
A, B = Extremum(f)
u = Vector((x(Corner[1]), 0), (x(Corner[3]), 0))
C = TurningPoint(f)
v = Vector((0, y(Corner[1])), (0, y(Corner[3])))
text2: "當x=" + (LaTeX[x(B)]) + " 時有極值" + (LaTeX[y(B)]) + ""
text1: "當x=" + (LaTeX[x(A)]) + "時有極值" + (LaTeX[y(A)]) + ""