(a-2分之1-a+2分之1）除以2-a分之2=[1/(a-2)-1/(a+2)]/[2/2-a)]=[(a+2)/(a+2)(a-2)-(a-2)/(a+2)(a-2)]/[-2/(a-2)]=[4/(a+2)(a-2)]/[-2/(a-2)]=-2/(a+2)
