HP 39gii oddity

[lrdheat]

Some functions seem problematic in home and plot mode. 3* (3 root X)^2-2*X presents some perplexing results in home and plot mode. I have latest firmware on physical and emulator. Roots are at 0 and at 3.375. In plot mode, the signed area works fine from 1 to any number of interest such as the root at 3.375. Yet, when attempting to find the area under the curve from 0 to 3.375, after awhile, it comes up with ~.08 vs. actual ~2.28. In home, the integral works fine for an integral beginning at 1 or higher, but for any starting point <1, say 0 or .7 or whatever, I get, after a fair amount of time, ER: invalid input (which I cannot find described in the full manual in it's discussion of error messages).

An earlier deleted thread on this equation surprised me when a person with the physical HP 39gii reported difficulties with the plot on this equation (area under 0-3.375), but in home, was able to get the ~2.28 for the integration instead of the invalid input error.

Any ideas on the erratic results...working fine on this equation in home and plot if my starting point is at or above 1, error message in home, incorrect result in plot when starting at 1? Bug? Am I doing something wrong?

Love how the points of interest are recorded in the named variable registers, and can be retrieved to be used in the home screen in equations!

[lrdheat]

As an addendum, with regard to calculus applications, the plot environment is far superior to the home environment in finding area under a curve or between multiple functions. It is successful in most cases, and is much quicker than in home. Accuracy is full 12 digit.

[kakima]

(05-15-2014 06:01 PM)lrdheat Wrote:  ... and is much quicker than in home. Accuracy is full 12 digit.

Using the Function app, plot the function FRAC(X) from 0 to 6.4 then find the signed area. My 39gII takes over forty seconds to deliver an answer that isn't correct to one digit.

Okay, so this is my favorite pathological example. It fails on every HP calculator with a built-in integrator, but it usually fails quicker than this.

In Home it comes up with ER: Invalid Result in under five seconds.