x19911007是什麼座 x^2=1/4*[1991^2/n-2+1991^(-2/n)]<br>所以1+x^2=1/4*[1991^2/n+2+1991^(-2/n)]=[1/2*(1991^1/n+1991^(-1/n)]^2<br>所以x-(1+x^2)^(1/2) =1/2[1991^1/n-1991^(-1/n)]-1/2[1991^1/n+1991^(-1/n)]<br> =-1991^(-1/n)<br>所以原式=[-1991^(-1/n)]^n<br>所以<br>n是奇數,原式=-1991^-1=-1/1991<br>n是偶數,原式=1991^-1=1/1991
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