Use the Formula $a \cos B x + b \sin B x = A \cos ( B x - C )$

Multiple Choice

Use the formula $a \cos B x + b \sin B x = A \cos ( B x - C )$ $\text { where } A = \sqrt { a ^ { 2 } + b ^ { 2 } } \text { and } \tan C = \frac { b } { a } \text { with } - \frac { \pi } { 2 } < C < \frac { \pi } { 2 }$ to determine the amplitude of f (x) . $f ( x ) = 2 \cos 4 x - 2 \sin 4 x$

A) $A = 11 \sqrt { 2 }$
B) $A = 9 \sqrt { 2 }$
C) $A = 2 \sqrt { 6 }$
D) $A = 2 \sqrt { 2 }$
E) $A = 2$