这两题都是凑等比数列求和公式
f(x)=1/(x^2+3x+2) =1/(x+1)-1/(x+2)
= 1/5 *1/(1-(x-4)/5) - 1/6 * 1/(1-(x-4)/6)
=1/5 sum((x-4)/5)^n) -1/6 sum((x-4)/6)^n
=sum[(x-4)^n (1/5^(n+1) -1/6^(n+1))
f(x)= (x-1 )/3* 1/(1-(x-1)/3)
=（x-1)/3 * sum(((x-1)/3)^n)
=sum(((x-1)/3)^(n+1))
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