Partial Differentiation Problem

[chesshaha]

Hi, I have two ti89s one old school, one Titanium.

Both calculators can't do partial differentiations right

Example: (source from http://pages.infinit.net/carl/TI89tutoen.html)

Compute the partial derivative fx of f(x)=sin(x*y) +cos^2(x+y)



But on my calculators, I get -2sin(x+y)*cos(x+y), which is not the right answer.

I have tried other example, and the calculator keeps getting it wrong.
And single variable differentiation calculations are working well, I did master clear on the calculator, and it does the same thing.

What's the problem? Please Help! Thanks!