Question 1172445
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The trig identity we can use is
{{{cos(A)-cos(B) = -2*sin((A+B)/2)*sin((A-B)/2)}}}


In this case, 
A = 1.4x
B = 0.6x


{{{cos(A)-cos(B) = -2*sin((A+B)/2)*sin((A-B)/2)}}}


{{{cos(1.4x)-cos(0.6x) = -2*sin((1.4x+0.6x)/2)*sin((1.4x-0.6x)/2)}}}


{{{cos(1.4x)-cos(0.6x) = -2*sin((2x)/2)*sin((0.8x)/2)}}}


{{{cos(1.4x)-cos(0.6x) = -2*sin(x)*sin(0.4x)}}}


<font color=red>Answer: Choice A</font></font>