find equation of tangent line calc?

f(x)=cos^2(x) at a=?/3

im using the m=lim(h->0) f(x+h)-f(x)/h equation.
im not sure what to do when i plug in f(x) into the equation. note: its cos^2(x) no cos(x)^2. is there a trig identity for this?
 
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