用公式求和：Sn=(x+1/x)^2+(x^2+1/x^2)^2+...(x^n+1/x^n)^2
问题描述：
用公式求和：Sn=(x+1/x)^2+(x^2+1/x^2)^2+...(x^n+1/x^n)^2
答
Sn=(x+1/x)^2+(x^2+1/x^2)^2+...(x^n+1/x^n)^2=x^2+1/x^2+2+x^4+1/x^4+2+x^6+1/x^6+2+...+x^(2n)+1/x^(2n)+2=(x^2+x^4+x^6+...+x^(2n))+(1/x^2+1/x^4+1/x^6+...+1/x^(2n))+2n=x^2(x^(2n)-1)/(x^2-1)+(1-x^(2n)...