I bought a new Casio FX-991EX today and tried to calculate some simple trigonometric functions.

When given:

1. x=pi/6 (In Rad):

16*cos(x)^6 - 8*cos(x)^4 - 16*cos(x)^4 + 10*cos(x)^2 - cos(x)^2

the result is:

** -1.8*10^(-13)**

2. x=pi/6 (In Rad):

16*[cos(x)^4]*[cos(x)^2] - 8*cos(x)^4 - 16*cos(x)^4 + 10*cos(x)^2 - cos(x)^2

the result is:

** 0**

3. x=pi/6 (In Rad):

16*cos(x)^6 - 8*cos(x)^4 - 16*cos(x)^4 + 9*cos(x)^2

the result is:

** 0**

The true value is 0.

All three expression are equivalent from the algebraic point of view.

I got the same results when I put x=30 (In Deg), so it is not Deg/Rad problem.

I got the same results when I used the FX-82ES.

I do not own anymore scientific calculators so I can not investigate this issue further.

I know that in this forum there are people that are more expert in Casio calculators than I am.

I want to hear your opinions.

thanks.

**Edited by jungingj, 16 October 2017 - 05:00 PM.**