Hello,
I've found a bug in Casio's fx-991ES calculator.

When you integrate\ calculate the derivative of a trigonometrical function USING DEGREES - the calculator brings a wrong answer.

for example I put this:

Integrate sin(x)dx from x = 0 to x =1
The calculator brings back: 8.726424738 * 10^-3.

The correct answer is: -cos(1)+cos(0) = 1.523048436 * 10^-4
also, Wolfram|Alpha brings the same answer(as me).

http://www.wolframalpha.com/input/?i=integ...g+to+x+%3D+1deg






In addition, I decided to check the derivative calculation of Casio's calculator. USING DEGREES.

I did this:
Derivative of sin(x) , at point x=1degree

the calculator's answer is: 0.0174506343

While the correct answer is:
cos(1) = 0.9998476952.

Also, Wolfram|Alpha brings the same answer as me:
http://www.wolframalpha.com/input/?i=deriv...+%2C+x+%3D+1deg


I checked it with a few more functions, and it still brings a wrong answer.
Though the calculator DO bring a correct answer while using Radians.

I looked in Casio Israel's website where can I report on this bug and haven't found yet,
if any of you can send me a link where I can report this bug, it would be wonderful.

Thank you , and be aware of this!

Hi talss,

It's easy to miss this but there is a note in the User Manual for the fx-115ES/991ES, specifying that the angular mode must be set to RADIANS for differentiation and integration.

Quoting from the manual, for integration:
"When performing an integration calculation involving trigonometric functions, specify Rad as the calculator's default angle unit."

and for differentiation:
"When performing a differential calculation involving trigonometric functions, specify Rad as the calculator's default angle unit."

It's easy to see how this is missed since you have to read all the little notes under the Integration and Differentiation headings in order to catch the fact that these numerical functions require that angles be measured in RADIANS. Hope this helps.

Jim C