This is a program that enables the user to input the coefficients of a cubic function and draw its graph. The cubic function
takes the form f(x)=ax^{3}+bx^{2}+cx+d. We need to create the cubic function with some modifications. The coordinates in computer screen starts from top left, therefore instead of x we used a certain number to substract x. As we define the width of the graph as 10, so we use 5-x so that the graph starts plotting from the center. In VB, the cubic function takes the following form:

**Private Function f(x As Variant)**

** f = a * (5 - x) ^ 3 + b * (5 - x) ^ 2 + c * (5 - x) + d**

**End Function**

To draw the graph, we use built-in function** PSet**.

In this program, you need to insert four text boxes for the user to enter the four coefficients, a picture box and two command buttons, one is to draw the graph and the other one to clear the inputs. Name the picture box as pic_graph, the draw button cmd_draw and the clear button as cmd_Clear. The runtime interface is shown in the Figure below:

Dim a, b, c, d As Integer

Dim x As Double

Private Function f(x As Variant)

' Need to make some transformation as the left coordinates in VB start from left

' our graph starts from center

f = a * (5 - x) ^ 3 + b * (5 - x) ^ 2 + c * (5 - x) + d

End Function

Private Sub cmd_Clear_Click()

pic_graph.Cls

txt_a.Text = ""

txt_b.Text = ""

txt_c.Text = ""

txt_d.Text = ""

End Sub

Private Sub cmd_draw_Click()

Dim y As Double

Dim w As Double

a = Val(txt_a.Text)

b = Val(txt_b.Text)

c = Val(txt_c.Text)

d = Val(txt_d.Text)

'Using a scale of 0.5 cm to represent i unit to draw the graph

' Need to make some transformation as the coordinates in VB start from top left

For w = 0 To 10 Step 0.0001

y = f(w)

pic_graph.PSet (w, 5 - y)

Next w

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