{【(a^2-4)/(a^2+2a-8)】/(a^2-4)}x(a^2-4a+4)/(a-2)
=【1/(a^2+2a-8)】x(a^2-4a+4)/(a-2)
=【1/(a^2+2a+1-9)】x(a-2)^2/(a-2)
={1/【(a+1)^2-9】}x（a-2)
=(a-2)/[(a+1)+3][(a+1)-3]
=(a-2)/(a+4)(a-2)
=1/(a+4)