Local Linearization of f(x) = cosx at a = $\pi$/2

Mirole
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f(x) = cosx, a = \pi/2
since, L(x)=f'(x)(x-a) -f(a)
f'(x) = -sinx
= -sin(\pi/2)(x-\pi/2) - f(a)


I'm stuck as to where to go next, is this even right?
 
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Nevermind, I got it to be -x+\pi/2, which is correct!
 
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