Excel VB Help needed

[jacamar21]

By whatever numerical means you prefer in Excel, fin any and all values of x for which the following tow functions f(x) and g(x) are equal to at least eight decimal places of accuracy:

f(x) = x^3 + .25x^4
g(x) = x + 2x*cos(x^2)

This is my problem, if anyone could give me some ideas to help me out i would appreciate it very much.

[ This Message was edited by: jacamar21 on 2002-05-19 12:12 ]

[Tommy Bak]

f(x) = x^3 + .25x^4
g(x) = x + 2x*cos(x^2)

x^3 + .25x^4 = x + 2x*cos(x^2)

x=0
-8*cos(x^2)+x^3+4*x^2-4=0

Insert that into a speadsheet and name a cell x

Now you use GoalSeek from the tools menu
and let that set the cell with the formula to zero by changing the cell named x
My result -0,000668646

Sorry I was too quick writing

regards
Tommy
(no garantee)






[ This Message was edited by: Tommy Bak on 2002-05-19 14:47 ]

[ This Message was edited by: Tommy Bak on 2002-05-19 15:18 ]

[Jay Petrulis]

On 2002-05-19 12:11, jacamar21 wrote:
By whatever numerical means you prefer in Excel, fin any and all values of x for which the following tow functions f(x) and g(x) are equal to at least eight decimal places of accuracy:

f(x) = x^3 + .25x^4
g(x) = x + 2x*cos(x^2)

This is my problem, if anyone could give me some ideas to help me out i would appreciate it very much.

[ This Message was edited by: jacamar21 on 2002-05-19 12:12 ]

Hi,

1. Graph the functions to get a rough estimate where they intersect.

2. Use Goal Seek as Tommy suggested to refine the answer.

3. Fix this UDF!!!

a) solve for x on one side of the equation
x = x^3 + .25*x^4 - 2*x*cos(x^2)

Type the right hand equation in a cell exactly as you see it.

call the following function SolveEquation(Cell from above)

Function SolveEquation(Equation As String, Optional Guess As Double)

Dim a As Double, x, lngCount

Equation = Trim(Equation)

Equation = Subst(Equation)



a = Guess

x = Evaluate(WorksheetFunction.Substitute(Equation, "xval()", "(" & a & ")"))



Do Until Abs(a - x) <= 0.00000001

    lngCount = lngCount + 1

    If lngCount > 50000 Then Exit Do

    a = x

    x = Evaluate(WorksheetFunction.Substitute(Equation, "xval()", "(" & a & ")"))

Loop

    

SolveEquation = a

End Function



Private Function Subst(func As String) As String

    With Application.WorksheetFunction

        func = .Substitute(LCase(func), "x", "xval()")

        func = .Substitute(LCase(func), "exval()", "ex") '(1)

        func = .Substitute(LCase(func), "xval()irr", "xirr")

        func = .Substitute(LCase(func), "xval()npv", "xnpv")

        func = .Substitute(LCase(func), "sumxval()", "sumx") '(2)

        func = .Substitute(LCase(func), "maxval()", "max")  '(3)

        func = .Substitute(LCase(func), "steyxval()", "steyx")

    End With

Subst = func

End Function

If this converges from your initial guess, great. If not, I don't know how to anneal it and will need some help.

So, this is not ready for prime time, but it may give you or others some insight in fixing this.

This same idea, if fixed, will apply to your Secant Method post as well. The advantage is that you can write a function almost exactly as you would on paper -- with explicit math operators -- and this will evaluate to the correct number.

Bye,
Jay

[ This Message was edited by: Jay Petrulis on 2002-05-20 17:50 ]