Which expression is equivalent to cos (1.4x) - cos (0.6x)

a.) - 2sinxsin (0.4x)

b.) 2sinxsin (0.4x)

c.) - 2cosxcos (0.4x)

d.) 2cosxcox (0.4x)

Cos (1.4x) = cos (x) cos (.4x) - sin (x) sin (.4x)

cos (0.6x) = cos (x) cos (.4x) + sin (x) sin (.4x)

The difference of these is

cos (1.4x) - cos (0.6x) = cos (x) cos (.4x) - sin (x) sin (.4x) - cos (x) cos (.4x) - sin (x) sin (.4x)

= - 2sin (x) sin (.4x)

This matches the first selection, ...

a.) - 2sin (x) ·sin (0.4x)