Differentiate f(x) = 2cos(-1) ((x) /(3) ) .
a) f'(x) = -2/(√(9 - x²) )
b) f'(x) = -2/(3√(1 - (x/3) ²) )
c) f'(x) = 2/(√(9 - x²) )
d) f'(x) = 2/(3√(1 - (x/3) ²) )