求lim(x->1)(1+cosπx)/(x-1)^2令t=x-1,则lim(x->1)(1+cosπx)／(x-1)^2=lim(t->0)[1+cosπ(t+1)]/t^2=lim(t->0)(1-cosπt)/t^2=lim(t->0)(π/2)^2*{2[sin(πt/2)]^2}/(πt/2)^2=(π^2)/2运算过程中,第二步1+cosπ(t+1)是怎么转化为第三步(1-cosπ

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