原式=[(a-2)/a(a+2)-(a-1)/(a+2)²]×(a+2)/(a-4)=(a²-4-a²+a)/a(a+2)²×(a+2)/(a-4)=(a-4)/a(a+2)²×(a+2)/(a-4)=1/a(a+2)=1/(√2-1)(√2+1)=1/(2-1)=1
