Ti-84 strange derrivative problem

[Bassie]

Hey there, I'm using a Ti-84 calculator to calculate derrivatives.
It goes like this:

Y1={the actual function}
Y2=nDeriv(Y1,X,X)

This seems to work, except for functions with cos(x), sin(x),..

Example:
Y1=X²
Y2=nDeriv(Y1,X,X)
Y3=2X

As Y3 represents the actual derrivative of Y1, Y2 and Y3 are the same.
But when I enter the following:

Y1=sin(X)
Y2=nDeriv(Y1,X,X)
Y3=cos(x)

As cos(x) is the derrivative of sin(x), Y2 and Y3 should be the same. But they aren't.
For X=3, I get:
Y1=0.258819045103
Y2=0.016858586
Y3=0.965925826289

Does anyone have an idea what I'm doing wrong? Thanks in advance.

E: I think I found the solution myself. It seems that X was considered as a degree value instead of a radian value!

 

[DrClaude]

As cos(x) is the derrivative of sin(x), Y2 and Y3 should be the same. But they aren't.
For X=3, I get:
Y1=0.258819045103
Y2=0.016858586
Y3=0.965925826289

E: I think I found the solution myself. It seems that X was considered as a degree value instead of a radian value!

It is important to make sure whether you are working with degrees or radians. But the results are not correct. Y1 and Y3 are consistent with X = 15°, not 3.