Estimate delta(x) using the Linear Approximation and use a calculator to compute both the error and the percentage error.

f(x)=cosx a=(pi/4) delta(x)=.06

delta(x)= ______
i got this part and it's -.04242

The error in Linear Approximation is:

and

The error in percentage terms is:

My book says that the error= the absolute value of [(delta(x))- [(f'(a))(delta(x))]].
I do not know if this formula is incorrect, or if i am not inputting the values correctly, but it seems that the error would equal 0, which is incorrect.... HELP!!