Solve for X

  • Thread starter Thread starter Kunal
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Kunal

hello all!

How do I solve the below equation in excel?

[5.2/(1+x)^2] + [12.1/(1+x)^3] + [16.7/(1+x)^4] + [16.8/
(1+x)^5] + [17.1/(1+x)^6] = 0


Thanks!

Kunal
 
Kunal said:
hello all!

How do I solve the below equation in excel?

[5.2/(1+x)^2] + [12.1/(1+x)^3] + [16.7/(1+x)^4] + [16.8/
(1+x)^5] + [17.1/(1+x)^6] = 0


Thanks!

Kunal
Click to expand...

One way might be to use the solver or goal seek.

If you have it try:

Tools - Goal Seek

Make 'x' be any given cell, and set your formula up so that it tries to make
the formula = 0 by changing 'x'.

HTH,

Alan.
 
Kunal said:
hello all!

How do I solve the below equation in excel?

[5.2/(1+x)^2] + [12.1/(1+x)^3] + [16.7/(1+x)^4] + [16.8/
(1+x)^5] + [17.1/(1+x)^6] = 0


Thanks!

Kunal
Click to expand...

I do not think your equation cannot be simply solved by excel because the
roots
appear to be imaginary, specifically
(-2.2629 + 0.813758 I), (-2.2629 - 0.813758 I), (-0.9005 + 1.2029 I) and
(-0.9005 - 1.2029 I).

a/(1+x)^2 + b/(1+x)^3 + c/(1+x)^4 + d/(1+x)^5 + e/(1+x)^6 = 0 may be
considered the same as solving for
a x^4 + (4 a + b) x^3 + (6 a + 3 b + c) x^2 + (4 a + 3 b + 2 c + d) x + a +
b + c + d + e = 0. As this is now a quartic, you could perhaps use an add in
such as
polynomials.zip from http://www.tushar-mehta.com/ ,which gives the results
as above.

mows
Excel XP SP2 / Win XP SP1
 
Hello Bernard,

I am interested in how you get your answer, as I cannot repeat it. As I see
it, the function

[5.2/(1+x)^2] + [12.1/(1+x)^3] + [16.7/(1+x)^4] + [16.8/(1+x)^5] +
[17.1/(1+x)^6] = 0

has no negative values and does not cross the axis, therefore has no real
roots, it approaches 0 as x-> + / - infinity. As x tends to -1 the functions
value becomes infinity.
As Dana says, setting x = 3277.8 (or indeed -3277.47308) the functions
value is 0.000000484. Choosing a higher x will return a value closer but not
equal to 0

Indeed, for large x the function approximates to 5.2 / x^2
For x approaching -1, the function is dominated by 17.1 / (1 + x)^6

I am not a solver expert, so possibly the easiest other way to tackle it is
to solve for the quartic

a x^4 + (4 a + b) x^3 + (6 a + 3 b + c) x^2 + (4 a + 3 b + 2 c + d) x +
(a + b + c + d + e) = 0

where a, b, c, d and e are the coeffs in
a/(1+x)^2 + b/(1+x)^3 + c/(1+x)^4 + d/(1+x)^5 + e/(1+x)^6 = 0

I suspect that it is now possible, with quite a bit of tedium, to find the
reducing cubic and so on using the likes of IMPRODUCT() & etc
from the Analysis ToolPak to eventually solve the quartic. The other
even simpler way is to cheat.
Use the addin from http://www.tushar-mehta.com/ I mentioned before!


Best regards
Peter -- (polygon moments / Greens theorem)

mows
Excel XP SP2 / Win XP SP1
 
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