△y=f(π/4+u)-f(π/4)=cos(π/4+u)-cosπ/4=-2sin(π/4+u/2)sin(u/2)
△x=u
y’=f’(π/4)
=(u→0)lim[-2sin(π/4+u/2)sin(u/2)/u]
=(u→0)lim[-sin(π/4+u/2)sin(u/2)/(u/2)]
=-sinπ/4
=-√2/2