Goal Seek Error in Excel 2013

[exceldoubt]

Hi,
I am using Excel 2013. I am trying to find the breakeven point in units using Goal Seek.
However, this issue occur.
Need help on this.

[gmr4evr1]

Maybe because the target value is 0, and the current value is -(negative?)$1500.00? Kind of hard to tell what's going on from a picture though.

[MrShorty]

Goal Seek is an implementation of the Newton Raphson root finding algorithm. I find that this is a good tutorial: http://www.cs.utah.edu/~zachary/isp/...ot/Newton.html It will show you how Newton's method works, and, important to the question at hand, when and why Newton's method fails to find a root. As gmr4evr1 noted, it is difficult to for us to draw any conclusions from a picture. Things I would suggest trying:

1) Try a different "initial guess". The tutorial shows how the choice of initial guess can impact the success or failure of the algorithm. Perhaps you simply need a better choice for your initial guess.
2) Newton's method needs a function to be "continuous" (I don't know if you math education extended into calculus and the definition of a "continuous function" -- might be worth looking into). Many functions, like IF(), COUNT(), COUNTIF(), and so on, tend to not be continuous, which makes them unsuitable for the NR algorithm. You may need to reformulate your function.
3) Many times, I find that I need to analyze the function in some detail. Generate several "y" values at several different "x" values and plot the results in a scatter plot. That will give you a "picture" of what the function looks like and can lend insights into why the NR algorithm fails.