Tough one for me

[X__man]

I can't figure out how to solve this one.

(2.8302) * (X^3) - (100 * X) - (3600) = 0

How would I solve for X?

Any help would be greatly appreciated.

[NotLKH]

Hmm,

Programatically, or generically exact?

[A$$Bandit]

-5.96-8.44i
-5.96+8.44i
11.92

[MXAlPhA]

yikes! you need 2 use the quadratic formula... maybe

I get:

(100(+/-) * i sqrt(30754.88))/5.6604

[sql_lall]

http://www.vbforums.com/showthread.p...hreadid=200157

I don't know if that will still be there when you get this, but that is another thread saying how to factor cubics in general.

Espescially:
http://mathworld.wolfram.com/CubicEquation.html

[krtxmrtz]

Another way to do it is by numerical methods. For example: for the real roots you can use the Newton-Raphson method which can find very accurate solutions and can be applied to a variety of equtions (it's especially suited for polynimes).



In the figure, the blue oblique line is tangent to the curve at the point x_i,f(x_i) so, the derivative of f(x) at that point is precisely tan(a). Thus, if you pick a convenient seed x₀ (i.e. located to the right of the sought root) you'll come out with a point x₁ which will be closer to the root. If you now draw a vertical line passing through x₁, and at its interception with the curve you draw the tangent line, this line's interception with the x axis will yield a value x₂ which will be even closer to the root.

Usually the method converges after only a few iterations. Applied to your equation you get the root:

x = 11.91871232 after only 5 steps (therafter my handheld calculator repats the values as it can't give any more digits).

For the complex roots, I believe the NR method can be easily extended to the complex field, but I have never worked with that

[krtxmrtz]

Sorry, in the above post the left formula has the denominator in reversed order. Should be x_i-x_i+1 actually.

[MXAlPhA]

what'd ya use 2 make that graph?

[krtxmrtz]

Originally posted by MXAlPhA
what'd ya use 2 make that graph?

MS PowerPoint + Inserted an "Equation" object + saved as gif

[MXAlPhA]

Originally posted by krtxmrtz
MS PowerPoint + Inserted an "Equation" object + saved as gif

i have to try it one of these days. I don't have an "equation" choice in my objects list in my powerpoint. I use xp by da way. Is it some addon or something? Some other program that does the equations?

[krtxmrtz]

Originally posted by MXAlPhA
i have to try it one of these days. I don't have an "equation" choice in my objects list in my powerpoint. I use xp by da way. Is it some addon or something? Some other program that does the equations?

MS Equation is an option when you install MS Office (you've got to check a checkbox at installation or re-installation time). Once installed it's available for Word documents, you can paste equations into Excel (not that they are of much use there, just embellish the whole appearance of your spreadsheet). More often than not the option is skipped by system administrators (or those who are responsible for installing software in offices or copmpanies) as not many people use it.

[MXAlPhA]

hmm... i thought i installed everything.