X^2一a^2=(X十a)(X一a) 1/(x^2一a^2) =((x+a一x+a)/2a)/(X^2-a^2) =1/2a×(x+a一x+a)/(X^2-a^2) =1/2a×((x+a)/(×^2-a^2)-(x-a)/(x^2-a^2)) =1/2a×(1/(X-1)一1/(X+1))
