Find the Differential of the Function at the Indicated Number $$f ( x ) = 13 \sin x + 4 \cos x ; \quad x = \frac { \pi } { 4 }$$

Multiple Choice

Find the differential of the function at the indicated number.
$$f ( x ) = 13 \sin x + 4 \cos x ; \quad x = \frac { \pi } { 4 }$$

A) $\frac { 9 \sqrt { 2 } } { 2 } d x$
B) $- \frac { 17 \sqrt { 2 } } { 2 } d x$
C) $\frac { 17 \sqrt { 2 } } { 2 } d x$
D) $- \frac { 9 \sqrt { 2 } } { 2 } d x$