a的n次方-a的n次方+1/a的n+2次方-3a的n+1次方+2a的n次方a的n次方-a的n次方+1_________________a的n+2次方-3a的n+1次方+2a的n次方

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a的n次方-a的n次方+1/a的n+2次方-3a的n+1次方+2a的n次方a的n次方-a的n次方+1_________________a的n+2次方-3a的n+1次方+2a的n次方

a的n次方-a的n次方+1/a的n+2次方-3a的n+1次方+2a的n次方a的n次方-a的n次方+1_________________a的n+2次方-3a的n+1次方+2a的n次方
a的n次方-a的n次方+1/a的n+2次方-3a的n+1次方+2a的n次方
a的n次方-a的n次方+1
_________________
a的n+2次方-3a的n+1次方+2a的n次方

a的n次方-a的n次方+1/a的n+2次方-3a的n+1次方+2a的n次方a的n次方-a的n次方+1_________________a的n+2次方-3a的n+1次方+2a的n次方

a^n-a^(n+1)=a^n(1-a)
a^(n+2)-3a^(n+1)+2a^n=a^n(a²-3a+2)=a^n(a-2)(a-1)

约掉a^n(1-a)
最后得:
-1/(a-2)

=a的n次方(1-a)/a的n次方(a²-3a+2)
=-(a-1)/(a-1)(a-2)
=-1/(a-2)
=1/(2-a)