I = ∫e^(ax)cosnxdx = (1/n)∫e^(ax)dsinnx
= (1/n)e^(ax)sinnx]-(a/n)∫e^(ax)sinnxdx
= (1/n)e^(ax)sinnx]+(a/n^2)∫e^(ax)dcosnx
= (1/n)e^(ax)sinnx]+(a/n^2)e^(ax)cosnx-(a^2/n^2)I
(1+a^2/n^2)I = (1/n^2)e^(ax)(nsinnx+acosnx)
I = [1/(a^2+n^2]e^(ax)(nsinnx+acosnx)
代入积分上下限，前乘以 1/π即得 an
分部积分，求出原函数。在计算
口诀 反，对，幂，指，三
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